Image Credit: Fine Homebuilding
More Musings of an Energy Nerd
Here at GBA, we regularly receive questions from readers about the best way to insulate a basement wall. Since these questions pop up frequently, it’s time to pull together as much information as possible on this topic.
In this article, I’ll try to explain everything you always wanted to know about insulating basement walls.
Is it worth insulating a basement wall?
If you live in Climate Zone 3 or anywhere colder, it’s cost-effective and wise to install basement wall insulation. This advice applies to those who live in most of New Mexico and most of Alabama, as well as all of Oklahoma, Arkansas, and South Carolina, and anywhere colder than these states. (Click here to see a climate zone map.)
Canadian researchers who studied basement insulation methods and costs in five Canadian locations (Toronto, Ottawa, Halifax, Edmonton and Victoria) concluded that “for all types and sizes of basements assessed in this study, the lowest total life-cycle cost was associated with basements insulated internally, full-height to a nominal level of R-20.”
How much money will basement insulation save you annually? According to a report published by the U.S. Department of Energy, the annual savings attributable to R-20 basement insulation in a 1,500-square-foot home ranged from $280 per year in Washington, DC to $390 per year in Buffalo, New York, assuming that natural gas costs $0.72/therm. (However, energy consultant Michael Blasnik cites two Minnesota studies that show lower levels of savings. See his 6/29/12 comments posted below.)
What do building codes require?
The 2012 International Residential Code requires basement insulation in Climate Zones 3 and higher. Here are the minimum code requirements for basement wall insulation — assuming that you are insulating with foam, not fiberglass batts:
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306 Comments
Heads up Martin
Unless my aging eyes deceive me, Martin, I think your text is referencing the CZ map GBA uses at the top of the Q&A section while the map you link to is kind of cart before horse.
Or am I the one confused this time?
Good summary by the way. Thanks. Makes me wish I had a basement.
Case study at Marc Rosenbaum's house
An excellent example presented over three blogs posts can be found at Marc Rosenbaum's blog "Thriving on Low Carbon":
http://blog.energysmiths.com/2011/07/basement-insulation-progress.html
Response to Joe W and Dustin Harris
Joe and Dustin,
Thanks for catching my error with the bad climate zone map link. The error has been corrected in the text.
is poly always a problem? Or simply near useless at best.
The idea of encapsulating wood and fiberglass behind poly certainly is scary, but I can imagine that concrete/foam/poly/stud/sheetrock might be acceptable....and (perhaps) even a very slight improvement.
Keep up the excellent articles. Good stuff!
(and I see you took care of the climate map link)
Response to Dustin Harris
Dustin,
Q. "Is poly always a problem?"
A. As far as I'm concerned, yes. You don't want it anywhere near the interior of a basement wall. If water beads up on one side of the polyethylene, it's going to stay there. Just say no to polyethylene.
savings overstated?
Martin- Lots of great information about how to insulate basements, but you may want to reconsider the savings claims.
I've seen a few studies of basement wall insulation and the measured savings are much smaller than the numbers you cite. You might want to check out this study in Minneapolis http://www.ornl.gov/info/reports/1991/3445606042537.pdf which was sponsored by ORNL and found average savings of 92 therms of gas for interior insulation which would be just $66/year using the same energy prices that were used in the source you cite. That source (also ORNL, strangely enough) claimed savings of $400 for R-10 in Minneapolis.
You might also want to check out this other Minneapolis study http://www.mncee.org/getattachment/c09d3ced-ab1f-49ae-be32-649adf7190f7/ where they found 197 therms average savings from interior insulation which would be $142 using that same energy price.
In both of these studies, the payback periods were fairly long even when excluding the costs for finishing the wall. One study did conclude that for intentionally conditioned basement the payback may be reasonable (in Minneapolis). But in the more common unintentionally conditioned basement, much of the reduction in conduction losses shows up as a warmer basement.
A basement wall insulation retrofit can provide lots of benefits -- including more comfortable and finished space, but the payback based on energy savings alone may not be that fast.
Response to Michael Blasnik
Michael,
Thanks for your comments and links. I have edited the text of my article to reflect your comments.
I welcome other GBA readers to share any links to relevant studies or monitoring data from their own homes or projects.
Comfort and Maintenance
I've designed and built homes with and without basement insulation in my builder days, and there are no comparisons on the comfort side, even though the ROI may not be there. Also, how much sheetrock and carpet mold will cost to fix when rigid foam is not installed?
Response to Armando Cobo
Armando,
Joe Lstiburek agrees with you -- he says, “Insulating the slab and basement walls controls mustiness and mold in the summertime. Now moisture doesn’t condense on cold surfaces in the basement. We insulate less for energy efficiency than for comfort and odor control.”
I knew it!!!
I don't want to brag here, but Dr. Joe is taking credit for my thinking.... ;-))
Martin,
Nice compilation ...
Martin,
Nice compilation ... thank you! Question ... which method, interior or exterior, would make most sense if the upper walls were framed with 2x6 construction, OSB, 2" rigid foam, housewrap, 1x3 furring and fiber cement cladding below which would be a 42" water table of synthetic stone? Is it best to align all of the rigid foam in 1 plane from footing to the roof truss before covering with housewrap; then attach lathe and cladding to the furring exterior to the foam? Trying to visualize what would work best ... interior or exterior insulation in this case. Would appreciate a sketch from someone as to how that might look from a wall section perspective. Thanks!
worry
I have no experience, but I would worry about vapor that does get to a concrete wall with foam over it (interior insulation). Won't that wall now be even colder than when naked, with no good way to dry? Unless your air sealing is perfect, some water will condense. What happens to it? Curious.
I'm with Klingel, interior
I'm with Klingel, interior rigid foam will get interior air behind which equals crap.
Better, spray foam or exterior...
Best ICF or Thermomass.
Response to Milan Jurich
Milan,
I'm not a fan of synthetic stone, which is hard to make look right. There are many pitfalls -- for example, the synthetic stone has to extend below grade to look right, and that sets certain limits on permissible details.
In short, you could install either interior or exterior insulation on your basement wall, but exterior insulation will probably be easier to detail in your situation. Good luck: synthetic stone over OSB is risky if you don't know what you're doing.
Response to John Klingel
John,
Concrete is a wonderful substance, and it holds up well when exposed to water. Remember, the exterior side of most basement walls and basement slabs are constantly damp -- and builders haven't been worrying about what that means for the last 100 years. You drive by concrete bridge abutments every day, and I doubt that you stay up at night worrying about the damp concrete. Nobody is running out with a hairdryer to keep the concrete bridge abutments dry every time it rains.
When a basement wall is covered with rigid foam or closed-cell spray foam, the interior air is no longer contacting the basement wall. It's a great system. There is no magical hidden condensation going on under the foam, because the interior air isn't there. The interior air is on the other side of the foam.
So now let's look at your comments.
"I would worry" -- please don't -- "about vapor that does get to a concrete wall with foam over it (interior insulation)." -- there really is very little vapor that makes it through the foam; that's physics. We can do the calculations so we know how small the number is -- "Won't that wall now be even colder than when naked" -- yes, that's the whole idea of insulation -- "with no good way to dry?" -- well, most concrete walls can dry a little bit at the top, especially if the above-grade portion of the exterior is exposed to some air. And remember, concrete likes to be damp, because that makes for stronger concrete. "Unless your air sealing is perfect, some water will condense." -- So now you are worried about moisture piggybacking on air rather than vapor that gets through the foam. But there is no air moving through the wall, because concrete is a good air barrier. So there isn't a stack effect or a pressure differential or wind operating to move any air. "What happens to it?" Well, if the concrete is damp, it's damp. But it's on the outside of your thermal envelope. The soil is damp too. Just stop worrying. You'll never make the environment outside your thermal envelope completely dry unless you move to Arizona.
Note to GBA readers
I won't be able to respond to questions posted on this page for a while, because I'll be on vacation for the next two weeks. I imagine that this dialog will continue, however, even if I can participate. I'll be back on July 16.
As John Brooks would say, 3D
As John Brooks would say, 3D networks, air will be getting past rigid foam sheathing. Spray foam, ICF, Thermomass much better.
Enjoy the vaca.... Shade is better than sunblock.... :)
Yes, true
Ok, so my suspicions have been confirmed. Yes, concrete likes being damp, and, no, I don't worry about wet concrete all over the country in bridges, etc; that stuff is not in my house. Too, I'm not worried about the concrete; it's the possible kooties. If concrete is damp, chit is going to grow on it. THAT is what I am concerned about. The fact that there won't be much air movement is a good point; not much air, not much water. Not much water, not much growing. OK; I quit worrying.... mostly. That said, were I to insulate a concrete wall on the inside, I'd make sure the humidity was low inside, just in case.
Sorry, Martin, but I STILL don't understand..
Since some moisture and air can get between foam insulation boards and create condensation, wouldn't spray foam be better? And wouldn't poly over foam boards = spray foam?
I certainly understand that one would not want anything encapsulated that has bad reactions with moisture. Also understand that spray is the only suitable solution on uneven surfaces like stone and mortar as air would circulate in gap behind foam boards.
Taming the basement
For any future basements, I would split every hair possible to achieve maximum perfection. I would strive to prevent any contact between the foundation concrete and the ground; and also between the foundation concrete and the interior air.
Concrete in contact with the ground will always have a moisture content from absorption of ground moisture. And if the interior surfaces of the concrete is exposed to interior air, the moisture in the concrete will always diffuse to the house interior if the interior vapor pressure is low enough, which is likely to be most of the time.
Preventing moisture from entering the exterior of the concrete would be adequate to stop this inward diffusing vapor, however, this is not the only source of moisture in the concrete. During high summer humidity, interior vapor is likely to condense on cool interior surfaces of basement concrete. This condensation will be absorbed into the concrete with a sponge action. It will tend to charge the interior zone of the concrete with perhaps a higher moisture content than what the outside soil is wicking into the exterior of the concrete. Then when the ambient humidity level drops a few days later, this inward-accumulating wetness in the concrete reverses itself and beings diffusing back into the living space.
So, in the summertime, you have moisture migrating through the basement walls from exterior ground moisture, and you have moisture periodically charging into the interior of the basement walls from the absorption of condensing ambient vapor from the interior. And then much of that periodic interior moisture charging comes back into the interior when ambient humidity drops.
Sealing interior surface of the concrete with an impermeable layer might solve the entire problem. It would prevent moisture in the concrete from entering the air of the living space. However, it still might be possible for highly humid interior air to condense on the surface of the interior sealing membrane. This may or may not be a problem, depending on the extent and whether this membrane was enclosed in a finished wall structure with limited air circulation.
Insulation on either the exterior or the interior might keep the wall interior temperature from falling below the summertime ambient temperature, and thus prevent condensation of highly humid interior air on the interior side of the basement walls. However, the insulation would need to be impermeable as is the case with closed cell foam.
I really do not know what microbial problems this basement dampness can cause, but I am convinced that it causes some. Concrete is a sponge, and we all know that a damp sponge is an ideal incubator for bacterial growth. I suspect that it is the interior air that transports the seeds of this microbial incubation to the moist sponge of the basement walls. In short, I don’t mind a basement, but I don’t want to live with one. So the key is to encapsulate the basement in such a way that it cannot absorb moisture from both inside and outside.
taming the basement
Well put Ron.
I put an apartment in my walk out basement in zone 6. I incapsulated and insulated the concrete inside and out. I did every detail right--except insulate under the footing--and I'm not losing sleep over that detail. The basement on average is 15% more humid than the upstairs and never goes above 55% unless you open the windows. But since I have an ERV, there's no need to open the windows. I did, however, use open cell icynene on the interior and sheet rocked over that. In retrospect, it might have been more prudent to use moisture and mold-resistant sheetrock on the walls--though I was smart enough to use metal studs. Now I guess I'll just hope there's nothing growing on that paper backing of the sheetrock. Time will tell.
what about the floor
Martin,
Thanks for your article about insulating basement walls. You hint at the benefits of insulating the floor. Can you give your thoughts for doing this in a completed home? Old DIY articles suggested a poly sheet on the concrete followed by 2x4 sleepers with the voids filled with foam and the whole thing covered with 3/4" plywood. I think this solution would cause a problem at the stairs. If carpet and pad are installed directly over the concrete, what advice do you have for the pad?
Thanks,
Don
Seal and insulate Basements
I am finishing up a total (daylight) basement remodel that will be living space. I excavated the perimeter, pressure washed the foundation, removed and reinstalled the footing drain, (filled with sediment from 1970), damproofed the wall, installed 2" closed cell foam board, covered the foam with painted galvanized metal roofing screwed with tapcons to the concrete (use impact driver, not hammer drill!) , the front third i am installing a nice rock veneer over mesh screwed through the insulation to the concrete on 6" centers. Inside on the west half i used existing furring strips but beefed them up to be 2.25 and infilled with rigid foam, then sheetrock. On the floor I installed warmwire, then schluter ditra and tile over. I was pretty pleased with the results except realized a thermo break on the floor would have been a benefit and i did not air seal above the walls, just used fiberglass insulaton.
Now three years later based on what i see (disturbing) in my new fluke infra red thermal camera, i am doing the east half inside. I removed all furring strips, installed 4.5" polysci insulation in the overhead joist to rim joist area, caulked all joints before the blocks then after the blocks were installed too. Applied 1" t and g closed cell foam board to the walls, then screwed (tapcons) thrrough the 2x4's flat against the 1" foam on 2' centers with 1.5 foil covered styrofoam (white stuff) in between the strips (makes a flush surface). On the floor i layed a strip of 3/4 " thick foam under the horizonal base 2.4 strip to keep it off the floor. Sheetrock will cover all. On the flloor I am using warm wire again but this time i am putting down 1/4" cork then the warm wire then tile. Oh and around the window framing inside I am putting 3/4" foam, then the trim.
It will be interesting to see the difference in the west and east sides after the floor is in. This project is at the 4000' level on Mt Hood in Oregon so we have snow on the ground from November to May.
Additions are better places
Additions are better places for added space verses basements which always are a compromise.
basement that has an exisiting vapor barrier
I have a basement that was previously insulated with rigid foam against the poured concrete wall but it also has a layer of poly under the drywall (my home is in Minnesota and where poly vapor barrier used to be a code requirement.) There is no evidence of mold or damp; should I be concerned? Should I consider removing the drywall and the vapor barrier and redoing it, even though there does not appear to be any problem?
Avoiding foam?
Martin, highly useful to have a clear resource for this tricky subject. But, a simple question: what if one wishes to avoid foams for cradle-grave or health concerns (for insulating/reinforcing existing basements)? You brush by this briefly, but what's the concern with a 2x4 stud wall installed 1" off the concrete filled tightly with ComfortBatt? What of using Roxul's new semi-rigid rockwool boards (ComfortBoard or CavityRock)?
Kathy, the poly is redundant
Kathy, the poly is redundant over the foam and is fine. Leave it.
Reply to Kathy Olmstead
Kathy,
Leave the poly vapor barrier. I cannot see how it could cause harm, and I do see a role for it. You did not mention whether the rigid foam you used was closed cell or open cell. It is true that closed cell foam would act as a vapor barrier, so in terms of the foam sheet alone, a poly vapor barrier would be redundant.
However, the foam sheets have joints between the sheets, and between the sheets and fir strips. If these joints are not meticulously sealed (I am guessing that they are not), vapor can pass through them, contact the cold concrete, condense there, and cause wetness that will fill the joints by capillary action, or simply be trapped between the back of the foam and the concrete. This wetness could damage the drywall and/or cause mold.
The poly vapor barrier will cover the joints on the warm side and prevent this problem. If I were insulating the interior of a basement with expanded polystyrene foam, which is a vapor barrier, I would still apply a poly sheet between the foam and the drywall as you have.
There is another point to consider, however. Moisture can wick through the wall from the exterior ground contact, depending on how the exterior is moisture proofed. If your interior insulation system blocks vapor, it can trap the incoming dampness inside of the interior wall system. So, in addition to preventing outward condensation of vapor, the proper system overall will also stop this incoming moisture absorption of the concrete by some type of exterior treatment.
I can certainly see a role for polyethylene sheeting both inside and outside of a basement wall.
Exterior insulation positives
Here in Maine we have done both interior and exterior insulation many times and have had good results. I will read the Minn. studies to see if they address the air leakage reduction when sealing up the rim joist in basements. We see very large heat savings with reduced air leakage in addition to the insulation when this is done right.
Two items I would like to add to the pros column of exterior insulation. It can; A.) protect your waterproofing/damproofing from the grinding of backfill. We use a rubberized, waterproofing membrane for concrete that requires a nice warm protector like XPS. And B.) it allows the interior to stay exposed (if left unfinished) to see any cracks or seams that have opened up.
interior and exterior insulation?
I purchased a 60 year old home last summer and quickly upgraded the insulation on the basement interior with XPS foam and a 2x4 wall. I'm now planning to excavate the exterior of the home to address some drainage issues and run my downspouts into a common footing drain. I know this exterior work should have been done before interior insulation but we were short on time for energy rebates and it needed to be done during the winter.
Outside, I hope to dig out around the foundation, installing a new footing drain and damp-proof the exterior concrete walls. I'm wondering while I have these trenches dug, is it still valuable to also wrap the exterior of the foundation in foam board too? Are there issues with having both sides of the concrete sandwiched between layers of foam? and will this exterior insulation still be cost effective?
Thanks for the help!
Don's question about floors
Can anyone answer his question? He asks about carpet directly over the slab -- I'd say no way to that -- but what about the poly over the slab, then foam board between sleepers covered by 3/4" plywood? That's best practice, no? I thought this article would address the floor and the walls at the same time.
To the website admins: I forgot my password and couldn't reset it using the reset password button. Something seems wrong there. I tried emailing you through the "contact us" links, but didn't get an answer. I made a lucky guess after numerous attempts and got it right.
Poly against a basement wall
I an a HERS Rater in MA. We have many builders who insulate finished basements with fiberglass batts. Why do you categorically proscribe the use of poly anywhere in such an assembly? It seems to me that a single layer of poly directly against the concrete would keep exterior moisture away from the studs and fiberglass. Painted wallboard inside would serve as an air barrier that would minimize the amount of interior moisture coming in contact with the potentially cold poly, and it would permit it to dry to the inside. Is this not better than batts in contact with the concrete?
What should I recommend to my clients who refuse to use anything but FG Batts in their above grade and below grade walls?
Foundation Insulation & Insects
In Western Pennsylvania, concrete block foundations are the most common kind. I have seen many cases where carpenter ants or termites have used the wall cavities created on interior side of the insulated basement walls as clandestine access to the interior. Many of the cross sections proposed here should at least be augmented with, for example; a thin, stainless-steel shield separating masonry wall top from the sill. This shield should be curved down over the top of the interior wall cavity and frame. A suspended ceiling could be installed and although it would conceal the shield, panels of the ceiling could be periodically raised to look for mud tubes or other evidence of an invasion.
Hairline cracks and other minor foundation defects commonly develop with time in many of these block foundations and the pests get through them. I think this issue, along with potential for complex moisture management oversights and those attendant problems, when combined with a relatively small documented monetary return by energy saving force me to remind you that basements are not living spaces. They should be well-lighted, well-equipped, ventilation-capable work, machinery and storage areas. In the last 20 years I've seen one atypical 'insulated' basement space in a high-end house that solved these issues by locating key service / mechanical / utility functions in an an approximately 4-foot wide 'hall' that separated the exterior sub- and above-grade masonry walls (on their interior side) from the insulated framed-wall side of the hall.
High Performance Basement Philosophy
Basement space is relatively expensive, but the cost is offset because you fundamentally need a foundation. There are cheaper foundations than basements, but they don’t offer any useable space. So once you commit to the cost of some type of foundation, it makes economic sense to add a little more cost and reap the benefit of basement space.
However, for average construction where cost restraint is king, the basement is often left as being merely adequate space for utilities, storage, workshops, etc. Bringing it up to the same quality standard as aboveground space would further increase its cost relative to the aboveground space. So basement space quality has always sort of hung in limbo. If more living space is needed, it often makes more sense to acquire it by expanding the aboveground space.
In reading a book by Mark White called Superinsulated Truss-Frame House Construction, Mr. White makes a convincing case that new houses should be built without basements, and use a concrete pier foundation instead. He cites a host of bad effects arising from basements.
I can see his point, especially when considering a high performance, superinsulated house where one is spending extra money to achieve a higher performance in several ways. With that objective, a substandard basement space is a greater contrast to the aboveground living space. Not only that, but the negative effects of the average basement space can spill into the highly perfected aboveground space and drag its quality down. A basement can be like an infection that makes the whole house sick.
I see no reason why the basement space could not be of the same quality as the aboveground space, and for high performance houses, I would work toward that objective for the basement. It would further increase the cost of the basement, but superinsulated houses are generally compact and space efficient, so the basement is relatively small, thus limiting the cost of its perfection.
However, the exact route to achieving this level of basement space perfection is something that deserves a great deal of study, particularly in the soil and drainage engineering of the surrounding ground. The whole zone extending from the roof overhang to the footings needs to be designed like a high quality machine.
Jason, the dryer the cellar
Jason, the dryer the cellar the more I say your idea is fine. Wet cellars should have no contents stored within and have no living space constructed. Better to build an addition or upgrade homes. I drylocked 2 coats an entire cellar including the floors and it made a huge difference. Dehumidifiers and sump pumps, etc. Add heat as needed. Control the humidity and temperature and much of the battle is won.
Mark Fredericks, don't
Mark Fredericks, don't combine gutters into footing drains. Big no no. They go their own way, away from the home. And there is no problem with foam on two sides of concrete. The only issue with foam is it getting too wet and also that ants, termites etc love it. ICF foundations are foam on two sides, they have information on their sites as to how to protect the foam. roof membrane, stucco etc.
Response to Dustin Harris
Dustin,
Q. "Since some moisture and air can get between foam insulation boards and create condensation, wouldn't spray foam be better?"
A. Perhaps -- although I don't think that properly taped or caulked rigid foam boards will allow air to get in contact with the concrete. Moreover, many builders are leery of spray foam because of the possibility of lingering odors.
Q. "Wouldn't poly over foam boards = spray foam?"
A. I'm not sure what you mean. Rigid foam boards are an effective air barrier and vapor retarder, so there is no need to use polyethylene if you have rigid foam.
Response to Don Mikulec
Don,
Q. "You hint at the benefits of insulating the floor. Can you give your thoughts for doing this in a completed home?"
A. Insulating an existing basement floor will rarely be high on the list of cost-effective retrofit options. But if you are aiming for the Passivhaus standard or willing to spend the big bucks required for a deep-energy retrofit, then it may make sense to insulate your existing basement slab. Here's a link to an article that discusses insulating basement slabs as part of a deep-energy retrofit: The High Cost of Deep-Energy Retrofits.
Q. "Old DIY articles suggested a poly sheet on the concrete followed by 2x4 sleepers with the voids filled with foam and the whole thing covered with 3/4" plywood. I think this solution would cause a problem at the stairs. If carpet and pad are installed directly over the concrete, what advice do you have for the pad?"
A. You can't install carpeting directly over a basement slab, even if you include a carpet pad, because of the risk of summertime condensation on the cold concrete. You'll end up with mold under the carpet if you install it on the concrete.
Response to Kathy Olmstead
Kathy,
Q. "I have a basement that was previously insulated with rigid foam against the poured concrete wall but it also has a layer of poly under the drywall (my home is in Minnesota and where poly vapor barrier used to be a code requirement.) There is no evidence of mold or damp; should I be concerned?"
A. No.
Q. "Should I consider removing the drywall and the vapor barrier and redoing it, even though there does not appear to be any problem?"
A. No.
Response to Jason Holstine
Jason,
Q. "What if one wishes to avoid foams for cradle-grave or health concerns (for insulating/reinforcing existing basements)? You brush by this briefly, but what's the concern with a 2x4 stud wall installed 1" off the concrete filled tightly with ComfortBatt?"
A. Roxul ComfortBatts are made from air-permeable mineral wool. The concern is that the insulation is incapable of preventing moist interior air from contacting the cold concrete. The result will be condensation that runs down the concrete wall and pools near the bottom plate of your wall.
Q. " What of using Roxul's new semi-rigid rockwool boards (ComfortBoard or CavityRock)?"
A. The same problem applies to these products, which are both air-permeable.
Response to Mark Fredericks
Mark,
Q. "I'm now planning to excavate the exterior of the home to address some drainage issues and run my downspouts into a common footing drain."
A. AJ Builder is right: you don't want to combine pipes that convey water from your roof gutters with your footing drain. If you do, you will be introducing more water to soil near your footings, and that's definitely not what you want. Conductor pipes that convey water from your roof gutters should be solid PVC pipe, not perforated pipe, and should slope to daylight or to a drywall far from your foundation.
Q. "Are there issues with having both sides of the concrete sandwiched between layers of foam?"
A. No -- it's done all the time with insulated concrete forms (ICFs). So if you want more R-value in your basement walls, you should definitely go ahead and insulate the exterior of your foundation when it is exposed for the repair work to your footing drains.
Response to Kevin Ring
Kevin,
Q. "Why do you categorically proscribe the use of poly anywhere in such an assembly?"
A. Because it is unnecessary and leads to moisture problems and mold.
Q. "It seems to me that a single layer of poly directly against the concrete would keep exterior moisture away from the studs and fiberglass."
A. It seems that way, doesn't it? But you are wrong. There is no way you can install poly against a concrete wall in a way that will prevent moisture from entering your basement.
Q. "Painted wallboard inside would serve as an air barrier that would minimize the amount of interior moisture coming in contact with the potentially cold poly, and it would permit it to dry to the inside."
A. It's almost impossible to install wallboard in such a way as to ensure that it is a perfect air barrier, especially if the wall includes electrical outlets and switches. Moreover, why do you think that the air between the wall studs and the air between your fiberglass batts will be dry? That is is basement air, and it's moist. There isn't any way to be sure the air in the wall is dry unless you install a miniature dehumidifier between each pair of studs. (Don't try this.)
Q. "Is this not better than batts in contact with the concrete?"
A. I don't like to play the "what's worse?" game. The fact is, you don't want to use polyethylene or fiberglass batts to insulate your basement wall, because we have ample experience in the field about how these materials perform. Sometimes they work, especially in a very dry basement, but very often they fail. And when they fail, they really stink.
Q. "What should I recommend to my clients who refuse to use anything but FG Batts in their above grade and below grade walls?"
A. You tell them, "I won't do any work that I can't stand behind, so I'm not going to install fiberglass batts to insulate your basement wall. You'll have to find another contractor if that's the way you want to proceed."
Response to Bo Jespersen
Bo,
Thanks for suggesting two other advantages to exterior foundation insulation. I have edited the article to reflect your useful additions.
Response to David Martin
David,
Q. "Concerning Don's question about floors: Can anyone answer his question?"
A. Sorry for the delay; I was on vacation. See my reply above (comment #38).
Q. "What about the poly over the slab, then foam board between sleepers covered by 3/4" plywood?"
A. That's one way. I prefer to install a continuous layer of rigid foam over the existing slab rather than interrupted strips of foam, however. For more information on insulating an existing basement floor, see:
Finishing a Basement Floor.
Green Basement Renovation.
The Stay-Dry, No-Mold Finished Basement.
The High Cost of Deep-Energy Retrofits.
Q. "I thought this article would address the floor and the walls at the same time."
A. No article can explain everything; the topic of this article is "How to Insulate a Basement Wall." If the links which I have provided don't answer all your questions, feel free to post further questions on our Q&A page.
re: what about the floor
Martin,
Thanks for your response. Also, thanks for the links in your response to David Martin and for the comments from other readers. Most of these solutions require the layering of insulation and plywood over the concrete floor. Will these require the indoor stairs to be redone? My basement has a walkout sliding door. Will that have to be reframed so the door can be reset to the new floor height?
Don
Response to Don Mikulec
Don,
Q. "Will these require the indoor stairs to be redone?"
A. If you want your stairs to be safe and have consistent riser heights -- then yes.
Q. "My basement has a walkout sliding door. Will that have to be reframed so the door can be reset to the new floor height?"
A. Yes.
your response and another question
Thanks for your reponses to my questions. I have one more: with insulation and plywood built up on top of the concrete, what is the likelihood of these drying should the basement flood?
Don
Response to Don Mikulec
Don,
If you have a plywood or OSB subfloor, standing water on the floor is always bad news, of course. If the flood was very brief, and you managed to clean it up immediately, the subfloor might be OK. But in most cases, you'll get buckling and delamination.
Did i miss something about polyiso being used "below grade"?
I was considering using two staggered layers of polyisocyanurate to insulate the interior of a new basement... but using two types of foam. Since I will not be able to get around to finishing my basement right away but will have to do that slowly over the next several years this will leave the foam exposed.
So, the layer on the interior will be the Thermax. The layer next to the cement wall would be Super Tuff-R (b/c it's cheaper per R and I'll be using it in a number of other places).
However, for Polyisocyanurate, the rigid foam board page on your site (https://www.greenbuildingadvisor.com/green-basics/rigid-foam-insulation) states: "... Because it can absorb water, polyiso is not recommended for below-grade applications. ..."
Isn't the interior of a basement considered "below grade"?
Do I need to use something else for the first layer (next to the cement)?
Response to Darek Jansen
Darek,
The information given in this blog is correct. As I noted above, "The best way to insulate a basement wall on the interior is with foam insulation that is adhered or attached directly to the concrete. Any of the following insulation materials are acceptable for this purpose: closed-cell spray polyurethane foam, XPS, EPS, or polyisocyanurate."
Thanks for alerting us to the unclear wording that you cited in another GBA article. That encyclopedia article has been corrected. It now reads, "Because it can absorb water, polyiso is not recommended for use under slabs or on the exterior of foundation walls."
Thanks for keeping us on our toes.
polyiso correction -Thanks
Thanks for clarifying (and so quickly).
As a web developer, I fully understand how hard it can be to reconcile information in different areas when entered at different times.
As a homeowner-embarking-on-a-large-addition, I'm just grateful I don't have to come up with a new plan.
Flogging a dead polyethylene horse
Martin, I'm sorry to say I'm still unclear on just why "Basement wall systems should never include any polyethylene." Here's my potential situation: I have an old house in Vermont with a fieldstone foundation/basement walls. They are leaky, and we've had the interior perimeter of the basement piped to the sump pump, with dimple mat extending about 4 inches above the floor. It has been recommended to me that we use spray foam insulation up around the sill and upper portion of the basement walls, which are dry areas to my knowledge. (The recommendation was that extending the foam all the way down to the floor would be overkill.) It has also been recommended that I put up a layer of radiant barrier bubble insulation (e.g., http://www.radiantbarrier.com/bubble-white-insulation.htm) against the wall, and apply the foam onto that, so the foam is not attempting to adhere directly to the potentially damp stone and old, crumbly-in-places mortar. This barrier would be tucked in behind the dimple mat at the bottom. It would be applied so that order of materials would be: earth, stone wall, aluminized side of bubble sheet, bubble interior of sheet, white polyethylene side of sheet, basement air or foam and then basement air, depending on how close to sill. This bubble layer would offer a small bit of insulation value, but the recommendation was made based more on its usefulness in enabling better application of the foam.
Any water collecting on either side of the bubble sheet should drop by gravity into the perimeter drain system, right?
What am I missing w/r/t moisture problems? Or is this use of polyethylene sufficient uncommon that you didn't have it in mind when making your recommendation?
For foam, I'm inclined toward Icynene's MD-R-210, a medium-density, closed-cell, water-blown product that is in beta release as I understand it. I am trying to avoid the more common spray foams given the concerns outlined by Alex Wilson at https://www.greenbuildingadvisor.com/blogs/dept/energy-solutions/insulation-keep-us-warm-not-warm-planet. I'm wide open to comments on the advisability of this choice.
Many thanks,
Jonathan
Response to Jonathan Teller-Elsberg
Jonathan,
You're on the right track.
As you can imagine, there are many ways to skin a cat. I'm not much of a fan of bubble wrap / foil products, however.
Many experts would recommend the installation of a plastic dimple-mat product on your wall, followed by spray foam. Others (like Joe Lstiburek) might install a layer of Grace Ice & Water Shield.
The idea is to be sure there is an air gap behind the foam insulation -- enough of an air gap that the water can find its way down to the French drain at the bottom of the wall.
It don't have any experience with the spray foam you are asking about. In most cases, high-density closed-cell spray foam is the type of foam to use in a damp environment. It has a proven track record.
Follow up on bubble wrap as barrier
Martin, thank you for your prompt reply. If you have more you think worth adding on why you aren't a fan of bubble/foil, I'd appreciate your comments -- certainly if they are relevant to this particular use. Given that you don't mention any deal-killing flaws with the bubble option, I'm inclined to stick with it for the following reasons:
1) The size of the bubble roll will make for easier installation (6 ft vs. 4 ft for Ice & Water Shield -- it's a low ceiling, so a 6 ft roll will allow me to do the job in one continuous sheet and no need for taping up seams). I'm confident in guessing that it will be much easier to install than dimple mat.
2) The bubble roll looks to be (very) slightly less expensive than Ice & Water Shield and, I'm confident in guessing, much less expensive than the dimple mat.
3) The bubble roll will provide a smidge of insulation value.
-Jonathan
This is great information but
This is great information but I am still worried about your concerns with polyethylene. It is actually recommended for the application you are describing on the following website. http://www.ecofoil.com/Applications/Basement-Walls-Insulation
Response to Brandi Borkgren
Brandi,
Since this is your third comment posted on GBA including a link to a site with R-value exaggerations, I'm beginning to realize that your are planting spam.
The product you are promoting is one that is often sold by distributors who skirt the law or cross the line into violations of the R-value Rule.
As I responded to the last time you posted a comment, I've written many articles on the topic; one of them was called Martin’s Useless Products List.
Here's what I wrote in that article: "Distributors of foil-faced bubble wrap 'insulation' have a rich history of exaggeration and fraud. A September 2003 exposé in Energy Design Update documented several wild exaggerations by manufacturers. Although foil-faced bubble wrap has an R-value of about 1 or perhaps 2, several manufacturers have falsely claimed R-values ranging from 5 to 10. In hopes of avoiding FTC enforcement action, the manufacturers, caught red-handed, sent EDU a comical cavalcade of apology letters. The bottom line: foil-faced bubble wrap costs just as much as — and in some cases much more than — 1-inch-thick rigid foam. As building scientist John Straube pointed out, 'I might recommend it if it were half the price of R-5 rigid foam, but if it costs more than R-5 foam then you have to be crazy or stupid to use it.' ”
Martin,
As a first time DIY
Martin,
As a first time DIY basement, I have been working on finishing my basement for almost a year. I have steel studs and tracks up for all soffits and walls. I have already run electric for lights and outlets thought the studs. I was getting to the point of insulation. I originally thought about using spray foam insulation, but with a price tag of $1800, I wasnt sure i wanted to put all my money into this system. The biggest concern with this system is the fact that some people suggest they can smell the foam in their house and it has ruined it for them. Also i dont understand how the slab of the floor isnt going to let in moisture. I don't plan on having XPS on the floor and then a subfloor.
Because I am limited with what I can do with the walls and electric already in place, about the most I can do outside of spray foam would be to use fiberglass insulation with kraft or foil backing or slide in 1/2" sheets of XPS behind the wall. I was leaning towards the 1/2" XPS behind the wall, but it is my understanding that in zone 5 of IL, that 1" at 5 R value is required?
Is it worth putting in the 1/2" or do i really need 1"? Do i even bother with this approach or just go with batting? I cannot tear down all the walls, it would be way to much to start over again. what about a product like Roxul
Read more: http://www.doityourself.com/forum/insulation-radiant-vapor-barriers/480588-basement-insulation-steel-framing-electric-done-need-help.html#ixzz2A8FFERVj
Response to Brian Eck
Brian,
I'm sorry to hear about your cart-before-the-horse problem. As you now realize, you should have installed a thick layer of rigid foam on your walls before you began installing your steel framing.
You won't get much thermal benefit from 1/2 inch of rigid foam. I don't recommend the installation of air-permeable insulation products like fiberglass batts or Roxul, because these types of insulation can allow warm, humid interior air to contact cold concrete (or the relatively cold surface of a thin layer of 1/2-inch rigid foam).
Here are your choices:
1. You could bite the bullet and pay for spray foam. (Even if you install spray foam, however, your steel studs are much too close to the concrete. You really want to have enough room for a thick layer of insulation between the cold concrete and the steel studs, which are conductors.)
2. You could disassemble your stud walls and insulate with a layer of rigid foam. Then you can put your stud walls back up.
So you believe that 1/2
So you believe that 1/2 ridged foam is not enough? How much xps foam is enough? The studs are generally greater then 1 - 1 1/2" away from the walls already. It is just very hard to put a 1" XPS foam board behind it with the studs already in place. I would feel comfortable with the spray foam application behind the studs. My biggest concern is the odor and or safety of it. Finding a qualified contractor in the chicagoland arean. To dismantle the studs would be a gigantic setback. Of over a years worth of effort.
Another response to Brian Eck
Brian,
You are in climate zone 5. The minimum code requirement for basement wall insulation in your climate zone is R-10. That means at least 2 inches of XPS.
Your steel studs are much worse thermally than wood studs; they act like radiating fins, conducting heat through any insulation you install between them.
Somehow, you need to get at least 2 inches of foam insulation between the back of your steel studs and the concrete wall, or it's hardly worth insulating. That is the minimum code requirement.
I'm not sure where you came
I'm not sure where you came up with R-10, can you elaborate? Im looking on this site and i see R-5, which is 1 inch foam.
Zone 5 R-5 for 2x4 walls; R-7.5 for 2x6 walls
https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing
Brian Doe's wall foam r-value question
Brian:
You are confusing the minimum thickness of foam on the outside of above-ground framing with the code required r-value for basement wall insulation using foam. The chart you cite is for the above-ground framing; the code cited by Martin is for basement walls.
How to insulate a daylight basement wall
Martin:
I am in Washington state, Maritime zone 4. I have a daylight basement which has a concrete stemwall ranging from 1 to four feet high, with 2x6 wood framing above that. Twenty years ago, when I built it, I insulated the exterior of the stemwall with 2 inches of foam but never finished the interior. Last year we hired a builder who framed an addition. He put 4 inches of foam on the outside of the new stemwalls, and an inch of foam on the outside of the plywood above grade, plus a rainscreen. On this new section, I was planning on leaving the interior concrete exposed, and after reading your article, that seems like a really good idea. But for the older sections that have two inches of exterior foam, we put an inch of foam on the inside and framed a 2X4 wall inside that which is still just exposed framing. My builder planned to blow fiberglass into the entire wall cavity, though I was contemplating using cellulose instead of fiberglass due to a few nasty stories I read here about blown-in fiberglass. But then I ran out of money.
Now we are trying to finish this project, but after reading your article I am a bit confused. It seems like where the stemwall is only a foot or two high, we might be able to get away with blowing in something since there are 6-7 feet of wood-framed wall above that will breathe. And the 2 inches of foam on the outside of the wall, combined with a winter ground temperature of 45-50 degrees one foot down, should result in the wall staying warm enough that it won't ever be dripping wet. Maybe. We do have excellent drainage around the foundation, and the exposed wall never looks damp. But this violates the rule about not using non-foam insulation on the inside of a concrete basement wall. And where the stemwall is four feet, it seems to me that there is a greater potential for condensation on the wall.
Other options: Should I cut off the 2x4 wall just above concrete height and isolate the concrete part from the wood framing above it, and then use only foam on the concrete part, leave the 2x4 wall uninsulated, and use whatever I want on the above-grade wood-framed wall? Or should I install a whole bunch of blocking in the 2x4 wall to isolate the concrete section below from the wood-framed section above, and then blow in insulation in the above-grade section while leaving the below-grade section of the 2x4 wall empty?
Oh, and next time? I would stick to insulating the concrete on the outside and leaving the inside surface exposed. When you use foam and a rainscreen on the wood framing above that, the transition to the concrete isn't so bad, the house is tight, and i wouldn't have to be scratching my head wondering how best to make this work now.
Thanks!
Response to Eitan Yanich
Eitan,
I think it's a good idea to avoid the use of fiberglass batts and cellulose on below-grade walls. However, not every expert agrees. Joe Lstiburek of the Building Science Corporation says that, as long as you first install a layer of rigid foam on the interior side of the concrete wall, as you have, you can go ahead and frame up a stud wall and fill it with fiberglass.
In most cases, that approach will probably work. Since basements sometimes flood, however, I think that fiberglass batts are still a bad idea.
It's your call how to proceed, but I think that your comments show that you have a good understanding of the relevant issues.
Extreme Climate Presents Challanges
Hi Martin,
First of all I'd like to thank you for providing what I believe to be an excellent source of information on insulating basements.
I have been struggling with air quality issues in my home for the last 5 years as a result of trapped moisture and mold growth inside the existing finished basement walls. I have asthma and a general heightened concern regarding the quality of air I'm breathing.
Your advise on the use of XPS and other types of foam makes sense and I have been looking for confirmation on similar ideas for a long time all while getting very mixed and confused feedback from local contractors.
I live in a very extreme Canadian Climate. In my climate zone the temperatures will fall as low as -37F for weeks at a time in the winter months, (late Oct- late April) and as high as 95F in the Summer months.
Inspecting the unfinished areas of my basement (only Fiberglas and poly over wood studs) I find a thick frost covering the concrete. In this same area there is a major draft at the bottom of said wall cavity as the air has begun to actually circulate as warm air is pulled down by somewhat of a reverse convection over the freezing cold concrete. It's a dramatic effect at the bottom of the wall where the poly is not quite sealed allowing the draft to travel over the floor with reasonable force!
The rest of the basement is finished with regular drywall over poly/wood studs and fiberglass against the concrete. I know there is something similar happening behind the drywall as I can smell a musty odor in the spring which would be when the ice condensate would start to melt.
It's not so bad the carpet gets wet but you can smell the dampness in the room.
To make matters worse my home is a raised bungalow. The concrete foundation walls extend roughly 5 feet above grade which makes for a large area of climate exposure to naked concrete. I can not insulate from the outside above grade as the design of the home wouldn't allow for it.
I have been considering tearing down the inside walls, and addressing any mold issues that may be present. The home is 20 years old and doesn't have any obvious foundation cracks or seepage issues. For the re-construction I like the idea of interior rigid XPS straight over the concrete sealed to the nines allowing very little chance of air meeting the concrete. I'd like to have spray foam professionally done on the top portion in between the floor joist header area after the XPS panels are installed for the rest of the wall seems like it would help seal everything up well.
What do you think of this combination of foam? Also, I would still like to use roxul inside the wall cavities that will make up the home theater area. This is strictly for acoustic treatment. I'm guessing no poly, just foam on the concrete then the roxul, then the mold resistant drywall ontop of wood 2x4's. What are your views on this?
Here my province the building code still specifies 6 mil poly over fiberglass / wood studs in basements below grade. Most of the new homes I have had a chance to see will have about 2" of ice and frost on the concrete underneath the poly, fiberglass. Obviously this is NOT the right thing to be doing.
Response to Anthony Strang
Anthony,
Your experience is a good confirmation of the reasoning that has led me, for years, to advise builders to avoid the use of fiberglass batts and polyethylene on below-grade walls. Your plan to correct the errors in your house makes sense.
Q. "What do you think of this combination of foam?"
A. Your plan to use XPS on the walls and spray foam on the rim joist area should work fine. Be sure that you meet your local code requirements for covering the foam with an ignition barrier or thermal barrier. If the foam is covered with gypsum drywall, you should be fine.
Q. "Also, I would still like to use Roxul inside the wall cavities that will make up the home theater area. This is strictly for acoustic treatment. I'm guessing no poly, just foam on the concrete then the Roxul, then the mold resistant drywall ontop of wood 2x4's. What are your views on this?"
A. That should work.
Q. "Here my province the building code still specifies 6 mil poly over fiberglass / wood studs in basements below grade. Most of the new homes I have had a chance to see will have about 2 inches of ice and frost on the concrete underneath the poly, fiberglass. Obviously this is NOT the right thing to be doing."
A. I agree. Considering your climate, this example of code stupidity is particularly glaring.
interior insulation on extrerior masonry above grade
Martin,
First off I like your style. Secondly, I have gutted the old 60's paneling in my upstairs 1940ish cape cod. Being a retrofit I obviously want too maximize my results for energy efficiency. Here's my dilemma; After tearing down the old drywall on the exterior brick wall the framing is only 2" in depth. I would love to use 2" rigid foam board over the whole wall and reframe the son of a gun on the inside of the foam board, but I don't want to lose and more sq footage or have to build the window sill out further. The main reason I ask is the framing is not true 16 on center.. Would metal studs make sense to save room? Should I cut the foam board and glue it to the brick in between the existing stud framing and seal up the edges and seams with "great stuff" spray foam or caulk ? Should I use Kraft faced fiberglass? Whichever one I use do I need a plastic over the studs and behind the drywall. There is no sheathing over the brick. I can easily touch and chip away excess mortar. The insulation that was under there is fiberglass batts with no plastic and the older wood is not rotted or any sign of mold.., Not sure how many wythes of brick. Not sure what kind of brick. Not sure if the brick is hollow or honeycombed. Its a larger block-like, ruffled edge, kiln dried clay brick, like 4" high 10" long and maybe 7“ or 8" deep. It is most peculiar. I live in Milwaukee WI. So its damn cold in winter and hot and muggy in summer, especially since I'm 6 blocks from Lake Michigan. Please share your thoughts and advise a lost novice.
Thanks,
Adam
Response to Adam Brindowski
Adam,
I don't think those are bricks. I think those are terra cotta block. More information here: Structural Terra Cotta.
If you insulate with rigid foam on the interior, it would be best if the insulation is installed in a continuous layer, not in small strips between the studs. Are the studs even necessary? If the only purpose of the studs is to provide a way to nail up the paneling, they can be removed.
Thermax interior insulation
When insulating interior basement walls for new construction in zone 5 (cold climate), is it acceptable to apply Dow Thermax along the inner poured basement wall from the footing to the the top of the wall. Original plan was to use 2" of XPS or EPS Type IX under the basement slab and 1" around the slab perimeter. Can the Dow Thermax replace the short 1" thick portion around the slab perimeter ... run it continuous from the footing to the top of the poured wall? If not, is it better than to apply the XPS/EPS at that area and then transition to the Thermax above the poured concrete slab?
Response to Milan Jurich
Milan,
If I were doing the work, I would use XPS or EPS for the vertical insulation around the perimeter of the slab.
You might be fine using polyiso in this location -- after all, the horizontal foam under the slab should keep your slab dry -- but why take a risk? The location is potentially wet -- so I'd stick with XPS or XPS.
When you come back later to install the polyiso on the interior of the concrete wall, be sure to seal the seam at the bottom of the polyiso with canned spray foam or a high-quality European tape.
Response to Martin Holladay
Thanks Martin ... appreciated!
how to decide when it is safe to insulate basement walls?
Martin, thanks for your blog and comments. I have bought a house which is now vacant and am improving it before moving in.
The first task was to install rigid foam in the rim joists - expanded polystyrene sealed with Fill & Seal and caulk.
A next task would be to insulate basement walls. However, there is a mechanical waterproofing system around the floor. It is a thin strip of metal conduit which works by collecting water and pumping it out. However, I don't see any signs of a water problem such as mold or discoloration.
I plan to add ground around the house and slope it away from the house for a "passive" solution.
My question is, how is it determined whether a basement is waterproof? Should I wait a couple years to make sure there is no water? I would lke to get this done before moving in but will be patient if that is best. Just curious what a knowledgeable person would do with a new used basement. Thank you.
Response to Erich Riesenberg
Erich,
I'm not familiar with the type of waterproofing system you describe, but it sounds like a variation on an interior French drain.
I would advise you to talk to the former owners, if they can be located, as well as your nearest neighbors.
Here in Vermont, the time for basements to get wet is April. If I want to know if a basement is wet, I wait until May.
I'm not sure where you live, but if you live in a cold climate, wait until the snow melts, and you have a big rainstorm. That will tell you something.
Thank you Martin. I will move
Thank you Martin. I will move on from the rim joist to the attic.
I may keep the "thermal mass" and do something else. Thanks for your blog and comments.
polyiso facings
I am trying to decide what type of rigid insulation to use on my basement walls and floor. I have been leaning towards XPS but am wondering about polyiso; my concern is what type of facing would be best against concrete., i have access to polyiso from a roofing project backed with black paper/felting (?) on both sides or foil on one side and the black material on the other. would it be best to use foil against the concrete? and xps on the floor for better compression strength?
thanks!
Response to Joell Solan
Joell,
As long as your basement wall has no signs of moisture entry or moisture problems -- a prerequisite for any plan to install interior wall insulation -- you can use polyisocyanurate insulation on your walls. The type of facing doesn't matter.
However, if you are installing rigid foam on top of an existing concrete floor, you should choose either EPS or XPS, not polyiso. For more information on retrofitting rigid foam above an existing slab, see these links:
Fine Homebuilding: The Stay-Dry, No-Mold Finished Basement
Fine Homebuilding Q&A: Finishing a basement floor
GBA Q&A: Basement floor insulation retrofit
GBA Q&A: Floating plywood floor
Green Basement Renovation.
The High Cost of Deep-Energy Retrofits.
post polyiso install dampness
Im at a crossroads here. I have recently installed 2" of what appears to be a fiberglass paper faced polyiso which I purchased ( firestone 95+ specd roofing 2nds) I spoke with the firestone tech and he saw no problem using this on below grade interior foundation walls. You also mentioned not to be concerned with the facing, which I was. So I proceeded. I left a 2" gap above the floor and prepared to sheetrock. I am noticing a heavy dampness on the botton 2-3 inches of the panel against to concrete. It wets my fingers. Concerned, I removed it to take a look. The rest of the panel and most of the wall were dry, but I am also noticing condensation in the walls to about 2-3 feet above the floor.
How should I proceed? I never had wall dampness before. Should I just replace with an xps?
My other thought was to place
My other thought was to place a 1/2" of tuff r or some foil faced, taped against the concrete and then reinstall the 2" polyiso?
Response to Chris van Wilgen
Chris,
Assuming that the polyiso was installed in an airtight manner (with the perimeter of each sheet sealed with caulk, construction adhesive, or canned spray foam), it really isn't possible for the moisture to be condensation. The source of the water may be your soil; the concrete may be getting wet from the exterior side. However, there is no easy way to diagnose the source of the moisture over the internet.
After you have removed one of the pieces of polyiso, I suggest that you perform a test to see if your concrete wall is damp. Tape a 12 inch square of clear polyethylene sheeting to the concrete wall in the area that worries you. Leave it there for a few days. If you see beads of moisture forming between the concrete and the poly, then the concrete is damp (and the moisture is coming from the soil).
How wet is too wet?
So I was wondering how dry a concrete basement wall must be in order to use the closed-cell spray foam? My basement weeps a little moisture (walls feel slightly wet to touch) maybe once every 2 years when we get a hurricane or numerous days of rain in a row. It only weeps like this in a few places (2-3).
Otherwise the walls are dry. Would even this limited amount preclude using the spray foam?
I'm just wondering because the cost/effort balance is pretty high for a few small leaks that only appear every 2 years or so...
A related question...can the spry foam contain the moisture in the wall or will the moisture push the foam off the wall? Is this a problem if no warm air is getting to the moisture?
Thanks, George
Response to George Heinrich
George,
My answer to your question is, "I'm not sure." I would advise you to ask your spray foam contractor, explaining the situation just as you have here.
I would also check the grade and slope on all sides of your foundation. Increasing the slope of your grade to encourage water to flow away from your foundation can solve many problems like yours.
Moisture Control
Nice post -- basements can be very difficult to insulate. And moisture control is one of the biggest factors. Leaks and cracks must be addressed first. Source control will keep the moisture from getting through the walls' cracks.
Installing a layer of dimple mat against the basement walls
Martin, You briefly mention a water control strategy is "•Installing a layer of dimple mat against the basement walls before insulating." This is something I am considering in a house with a poured concrete basement with 8' walls and grade at 6'. The dimple mat should terminate at grade level. What would be acceptable methods of insulating this wall? Should I be worried about moisture condensing on the membrane between the insulation. We have the classic cold Canadian climate. The most cost-effective insulation would be fibreglass batts against the membrane and maybe some XPS against the top 2' of concrete. What would you recommend in this situation.
Response to Mike Wall
Mike,
If you are installing the dimple mat on the interior, you might as well bring it all the way to the top of the wall, to make the surface you are insulating co-planar.
The only two insulation types to consider (as I wrote in my article) are spray polyurethane foam and rigid foam. Don't use fiberglass insulation on the interior of a basement wall.
If your basement has a water entry problem, you need to install an interior French drain, connected to a sump, at the base of the wall before installing the dimple mat.
Why use equal insulation thickness on unequal heat loss?
From the reading I have done, the heat loss in a basement wall is predominantly near the top, and it tapers off greatly as the wall descends into the ground. Since multiple layers of foam with staggered seams are a good idea anyway, it seems logical to install less insulation on the lower parts of the wall and more on the upper parts (for example, 2" on the bottom 1/3, 3" on the middle 1/3, and 4" on the top 1/3). It should yield less heat loss for the same foam budget.
Conveniently, this would also leave more room on the lower parts of the walls for electrical boxes, allowing you to use thin wall framing such as 2x4s on their sides, without having to dig out holes in the foam.
Is this as good of an idea as it seems to me?
Response to Nick Welch
Nick,
You can take this approach if you want. The trouble with the approach is that heat doesn't just move through the concrete horizontally; it also moves through the concrete vertically. Concrete is an excellent conductor. It acts like a thermal bridge.
The most effective and easiest way to address all of these issues is to follow the code (or do better than the code) and install a uniform layer of insulation on your basement walls. You won't regret it.
Cognitive dissonance re condensation
Martin,
I'm struggling with a bit of cognitive dissonance regarding the use of vapor barriers in basement and condensation at the interior boundary of the concrete foundation.
If it is achievable to seal a piece of polyiso well enough to prevent condensation at the concrete-PI boundary, why isn't it achievable to seal a sheet vb such as polyE against the concrete? Or to use a paintable VB? If so, why wouldn't this allow the use of a batt product such as Roxul ComfortBatt? I'm guessing you are going to say because Roxul is air permeable and needs an air/vapor barrier on the interior to prevent interior-generated moisture from entering the stud cavity. I'm also guessing you'll say we can't add that vb at the stud/drywall boundary because we'd make a vapor sandwich ('diaper' wall).
To which I ask a followup question: if we use PI at the interior concrete boundary, then won't any interior vapor barrier at the drywall/finished surface create a diaper wall?
I'm going to post a question with more details in Q&A but thought that my question re VB placement/avoiding double VB would be useful to some people reading this thread.
Response to Keith H
Keith,
The glib way to answer your question about using a polyethylene vapor barrier to keep concrete walls dry would be, "The reason we know that it doesn't work is because thousands of builders have tried it, and thousands of remodelers and home performance contractors have had to clean up the moldy mess caused by these failures. We know it doesn't work because of field experience."
However, a more thoughtful response would explain that fiberglass batts and Roxul batts contain air. The air between the fibers is basement air, and it contains moisture. It isn't some kind of miracle, dessicated air. So condensation is possible. Polyisocyanurate sheets have no air; the only air trapped is a very small amount of air near the seams and between the foam and the wall.
The air between the fibers of the fiberglass or Roxul insulation is not divorced from the basement air. As temperature conditions change, there is a pumping action that creates air exchange between the air in the insulation and the air in the room. When the air is warm, it expands. When it is cool, it contracts. As this happens, pinholes in the poly allow for air exchange. This process introduces new moisture into the insulation cavity.
Question about finishing a basement in a new home
I am currently looking at finsihing up my basement in my 3 year old home. After much research along with this article I am convinced that these are the best practices to follow. However that being said, my basement is very dry and no issues (that are noticeable). I live in a very harsh climate where it can get to -30c in the winters.
The basement was left with R12 fibreglass from floor to rim joists stuffed in between framing along all the walls , pressed directly on the concrete wall. A loose sheet of poly is stapled along all the framing.
So my question is what do you recommend going forward to effectively finish these walls?
1) Remove all poly, fibre and framing and start from scratch following the practices as indicated in the article
2) Remove the fibre and poly and fill in the 2 foot sections in between the framing with rigid insulation and foam all the edging within the framing. However this would mean the framing is still up against the concrete directly too.
In either option can I reuse all the R12 and put on top of the rigid insulation again (with no poly) and cover with drywall without issues or just toss the fibreglass?
Any feedback is greatly appreciated
Response to Aaron De Boer
Aaron,
The safest approach is to remove the polyethylene and fiberglass, and remove (or loosen) the stud wall components so that the stud can be moved inward, creating a gap between the studs and the basement wall. That would make it possible to install closed-cell spray foam.
However, all that work is daunting (and potentially wasteful). If you don't find any dampness or mold behind the fiberglass, you could adopt another approach: watchful waiting. If your basement is so dry that you don't have problems, you could leave well enough alone. It's your choice.
An issue with one wall
Hey, this is a great article with some great questions and answers. Thanks for taking the time to do this.
I have an issue that I would greatly appreciate some input on. I am in the process of finishing my basement. I've adhered 2" foam board around nearly the entire foundation perimeter. The 2x4 framing is about 2 inches from the foamboard and I figured I'd add roxul insulation between the studs to help additionally with climate comfort for the northeast winters.
My problem is the one wall (about 7 feet wide) is right next to the stairs coming down to the basement. I only have 1 inch between the stair stringers and the concrete wall. I can't get even 1.5 inch foamboard in there so I was going to put up some furring strips, cover with heavy mil plastic and then apply a mold safe sheet rock to the strips. Is that acceptable or do I have other options?
Pete
Response to Pete R.
Pete,
If I were you, I would slide 1 inch of rigid foam between the concrete wall and the stair stringer. Your other option is to remove the treads and slide the stringer out a little bit further from the wall. Then cut back the treads and reinstall them.
Roxul section comments
Martin - Can you comment on any pitfalls of the recommended basement wall section outlined in the Roxul Comfortboard IS Basement Application Sell Sheet? The section shows a concrete foundation wall, then an "Air/Moisture Barrier", then the Roxul Comfortboard, then stud wall with Roxul batts, then a vapor barrier, then the drywall...
http://www.roxul.com/files/RX-NA_EN/pdf/Sell%20Sheets/ComfortBoard%20IS%20Basement.pdf
I've been researching this topic for several months and had landed on this basement wall section, until I read this article.. But then I think twice that the Roxul team would recommend a section that has been a proven to failure... Do you feel this section is different enough from older diaper wall/poly sections that it could survive? I'm in St. Louis, MO, Zone 4A / Mixed-humid. Currently I have rolls of CertainTeed Membrain to use both against the concrete and behind the drywall, and lots of Roxul on order. I'm more concerned first about the mold issue, second about R value.
Additionally, the back wall of the basement is a studded wall (Walkout basement). Based on your article, would you also not recommend poly (CertainTeed Membrain in my case) on this wall section? The walkout wall is vinyl siding, Tyvek wrap (lapped and Tyvek-taped properly by me one night after the builder got it wrong), 1/2" insulated sheathing, 2x6 studs.. See attachment.
Attached are the wall sections I have planned currently, but after reading, I'm wondering if I should scrap a lot of that and go with the rigid foam?
I certainly appreciate any commentary. I really try to avoid this type of pointed question, but in months of searching I've found so many conflicting articles and so many just seem to simply be outdated... I was surprised to read the Lstiburek post you link to where he admits some of his older advice is obsolete... Which is why I hope my question can clear up a lot of questions for not only myself, but for other readers as well.
Response to Randy Cook
Randy,
I hate to be blunt, but Roxul is in the business of promoting the use of their products. In that respect, they don't differ from any other insulation manufacturer. They want to sell more mineral wool.
If you have a dry basement with no water entry issues and perfect interior drainage, and if the interior relative humidity of your basement stays low all year round -- including during the summer -- it's perfectly possible to use fiberglass batts or mineral wool to insulate the interior of your basement walls. Plenty of people have done it, and if your basement has the conditions I described, fiberglass or mineral wool can work well.
The problem is that many basements aren't as dry as the one I described. Once we're dealing with a basement that is occasionally humid, we're stepping into a new territory, and the use of interior air-permeable insulation becomes risky.
The Roxul brochure you linked to isn't based on building science. The brochure specifies the use of an "air/moisture barrier" (material not specified) between the Roxul and the concrete wall; but there is no need for an air barrier at this location, because concrete is an air barrier. If polyethylene is used, it's very possible that moisture will condense on the interior side of the poly when the interior air is humid. This moisture will dribble down the poly and puddle near the bottom plate.
Determining which material to use depends on many factors: not only on cost, but on how much risk you want to assume. If you're like me, and don't like risk -- and you don't want to tear out your basement walls in 6 years to fix them -- I suggest that you use rigid foam or spray foam to insulate the interior side of your basement walls.
What about terracotta, also frost failure from insulation?
Great article, for retrofitting insulation in existing home, what is the reasoning behind the statement "If your basement has stone-and-mortar walls, you can’t insulate the walls with rigid foam. The only type of insulation that makes sense for stone-and-mortar walls is closed-cell spray polyurethane foam."? Does this apply to structural terracotta foundations as well? Does this have something to do with moisture? Could spray cellulose work?
Also what about frost failure (freezing of wet soils, which push into foundation causing cracks) when foundation is insulated, is this a problem or myth?
Response to K.T.
K.T.,
The reason that you can't install rigid foam on the interior side of a stone-and-mortar wall is that the stone wall is too uneven and bumpy.
The advice given for stone-and-mortar walls does not apply to terra-cotta walls. If the terra-cotta wall is even (co-planar), there is no reason you can't insulate it on the interior with rigid foam.
Spray cellulose insulation should not be installed on the interior side of a foundation wall, for all of the reasons listed in my article. This is a damp environment, and cellulose doesn't handle water well. Cellulose needs to stay dry.
Decades of experience has shown that insulating basement walls will not cause walls to fail due to the freezing of the soil. Needless to say, it's always a good idea to manage the water on the exterior side of your foundation. Normal practices call for some mechanism to handle water that is shed by your roof and to handle rain (for example, by proper grading and the use of swales). It's never a good idea to have saturated soils near your foundation walls. For more information on these issues, see Fixing a Wet Basement.
One more comment on frozen soils and foundation walls: in New England, many barns have basements. Although the soil around these barns freezes, the barns have stood for 150 years.
Basement with batt insulation and having moisture problems
This article was extremely helpfull by the way. My wife and I live in the lehigh valley in pennsylvania. Our basement has the batt insulation installed on the entire interior basement walls.. This insulation was installed by the contractor shooting the nails into the concrete to affix the insulation to the concrete. Currently our winter temperatures have been in the twentys for about 3 weeks now. Last night I was in the basement and noticed that were the insulation does not cover the concrete there is moisture or almost ice buildup forming on the concrete. Also what I noticed is that the batt insulation does not go all the way to the basement floor there is about a 4 in. gap and in this gap there is condensation forming.
Note: Half of my basement is above grade, the condensation is only on the above grade areas.
So what is the recommendation to help with this problem.
I was hoping to start to finish my basement next year but don't want to start this project till the moisture issue is taken care of.
Do I need to remove the batt insulation in the above grade area and replace with foam board?
Martin: Please add info on fire stops!
Thanks Martin for another helpful article and follow-up on basement insulation.
Please add info on firestops for basement walls, however -- which are required by the 2012 IRC and most inspectors. Foam board and spray foam that runs along a concrete wall, for example, must be interrupted every 10 linear feet by a firestop, e.g. a PT stud that is mounted and sealed tightly against the concrete wall, or a gap that is packed tightly with mineral wool. Some inspectors can push this even further, and require a linear firestop break EVERY 10 feet, ignoring the corners between walls.
The top of the interior stud wall also need to be firestopped, e.g. with a continuous strip of 3/4 inch OSB that bridges the gap between the top plate of the wall and the mudsill -- or pack the gap tightly with mineral wool or unfaced fiberglass. Some inspectors will not approve mineral wool or fiberglass, and prefer solid blocking. Ask your inspector before you start.
Holes in the studs or top plate, e.g. to run electrical wiring or pipes, must also be filled with fire resistant material, e.g. 3M Fire Barrier sealant.
Here are a couple of helpful resources:
http://contractorkurt.com/2012/12/31/how-to-firestop-your-basement/
http:/www.jlconline.com/codes-and-standards/fire-blocking-basics.aspx
We also ran into a problem with a commonly used "firestop" product: the orange "Fireblock" version of DOW Great Stuff foam sold at Home Depot and Lowes. I was surprised to discover that Fireblock Great Stuff ignites at 240 degrees, and DOW does NOT recommend it as a "firestop"!! Here is a quote directly from the DOW website:
"This product is defined by the International Building Codes as the use of approved building materials installed in concealed spaces to resist the migration of fire and hot gases. GREAT STUFF™ FireBlock is tinted orange to be more recognizable to building code officials. This product is not approved for use in firestop systems."
The third sentence contradicts the first two, and the label on the can. (Source: http://greatstuff.dow.com/products/fireblock/ See the "reviews" option.)
I contacted DOW ans asked them for clarification, with no luck. With the clout of GBA and the Taunton Press, maybe you can convince DOW to reply.
In summary, this "basement insulation" guide needs to be updated -- and a new guide to firestops would be very helpful to your readers. There is a lot of confusion out there.
Thanks for all of your good work!
Mark
Response to Mark Hays
Mark,
Thanks for your suggestion. I have edited my article to include some information on fire blocking.
Interior basement insulation advice
Martin,
Thank you for your research and faciliting this good discussion. I installed an interior wall insulation system 4 years ago to take advantage of govnt rebates, but after further consideration, I may have been too hasty. I am at the start of finishing the basement (currently just studs and insulation), and I am considering a change to the insulation system.
I live in southwestern Ontario with sub-zero temperatures from Dec-Mar, reaching -40C with windchill.
Here is the current arrangement:
1. Poured concrete wall
2. Typar house wrap. Installed with white side facing the wall: moisture movement from the concrete wall into the house is reduced, but there is stil moisture permeability from the interior towards the wall. Not sure whether this was necessary.
3. 0.5" rigid board XPS. Sealed all seams with Tuck tape.
4. Spray foam insulation in the headers, and about 24" below the
headers, on top of the rigid board XPS.
5. 2x4 studs with a 2" air gap between the studs and the XPS.
5. Roxul exterior wall, moisture-resistant batts for 6" studs on 16" O.C. (R-24)
I am using 1.5"x3.5" SPF wood studs, fastened with tapcom concrete
screws, 2" from the XPS, to give a 6" wall-to-front-of-studs gap.
The bottom sill plate is fastened on top of the sub-floor which is
made of 3/8" dimple wrap and 1/4" OSB plywood.
I am considering the following changes:
a. Remove the Typar house wrap
b. Add 1.5" of XPS for a total of 2" of XPS. Stagger the install and seal with tuck tape.
c. The XPS is not fastened against the wall. Since I'll add another layer of XPS, I'll use screw/washer fasteners to secure both XPS boards against the wall.
d. Install the studs against the XPS to remove the air gap
e. Reseal any gaps with two-component spray foam
f. Cut off batt insulation in the vertical plane to reduce compression on installation.
g. Building code in my area requires a vapour barrier on the warm side, so I am considering the MemBrain technology, even though I would prefer to install nothing.
h. Replace the OSB subfloor with plywood
What are your thoughts?
Thank you for your time,
matt
Response to Matt M
Matt,
Most of the steps you suggest are sound.
1. You can leave the Typar in place if you want. It isn't doing any good, but it isn't doing any harm. You should know that Typar and Tyvek are both vapor-permeable in both directions, so it won't really stop water vapor transmission.
2. You could try to convince your local inspector that the proposed XPS is already a vapor retarder / vapor barrier. If the inspector won't listen to logic, then I guess you will have to buy the MemBrain.
Zone3,Georgia, ? Basement wall insulation
Hello Martin,
Thanks for your time and efforts. Very informative website.
I am currently trying to finish my walkout basement. However, I already have studs up, 2x4, electrical and plumbing ran. My issue here is how do I insulate this basement. I bought the house 4 months ago, and have not seen any water issues, slight condensation on windows though.
I cannot afford spray foam, I only have about 1 1/2 behind studs.
1. Should I try to put 1" rigid foam xps behind stud, seal, tape as much as I can, then fiberglass, then drywall?
2. If I fit in 2" bwtn studs, seal as much as I can with greatstuff? If I do this, being that 2" has a r-10, do I even need to use fiberglass? Now this is all against concrete walls.
3. How do it insulate the non-concrete walls ,( the walkout side) if I decide to go with either method? Is it the same as nonconcrete walls. It's already studded.
Now while changing insulation at rim joist, the area in the basement under the stairs, there are some rotted plywood. I can see the brick of stairs. Apparently I was told no flashing was use. Some repair to the stairs was made not too long ago, it seems. How can this situation be rectify? I have someone coming out to check it out, but I just want to ask the right questions.
Also it's been hard to find 1" inch pink stuff or dow in my local area. But HD sells 3/4 inch of owens corning rigidfoam sheathing. Can I use this stuff indoors? I guess the difference is the plastic on it to protect against the elements. I don't have time to order online.
Thanks again, Natasha
Response to Nat (Natasha) Hilton
Natasha,
If I understand correctly, someone has framed 2x4 walls near your concrete foundation walls, leaving a gap of 1 1/2 inch between the concrete and the 2x4s. Is that right?
If that's correct, then the best way to insulate these walls is with closed-cell spray polyurethane foam. The installer should take care to make sure that the space between the concrete and the studs is completely filled.
If you can't afford to do that, you can try to slide 1 1/2-inch-thick XPS insulation between the concrete and the studs. This job needs to be done in an airtight manner. It's going to be awkward -- much more difficult than spray foam. Every seam in the XPS needs to be carefully sealed with canned spray foam or high quality tape.
After that work is done, I suppose that you can install rigid foam between the studs using the cut-and-cobble method if you want. I'm not a fan of cut-and-cobble, but you can do it. Again, spray foam would be easier. For more information on the cut-and-cobble method, see Cut-and-Cobble Insulation.
Q. "How do it insulate the non-concrete walls ( the walkout side) if I decide to go with either method?"
A. Just as you would any other above-grade wall. Whatever method you choose, pay attention to air sealing.
Q. "Now while changing insulation at the rim joist, the area in the basement under the stairs, there are some areas of rotted plywood. I can see the brick of stairs. Apparently I was told no flashing was used. Some repair to the stairs was made not too long ago, it seems. How can this situation be rectified?"
A. I don't know if you are talking about interior stairs leading to the basement or exterior brick stairs leading to your front door. In any case, it's impossible to diagnose this problem from your description. This problem requires a site visit. If you have rotten sheathing, a rotten rim joist, or missing flashing, you may have to do extensive repair work.
If I understand correctly,
If I understand correctly, someone has framed 2x4 walls near your concrete foundation walls, leaving a gap of 1 1/2 inch between the concrete and the 2x4s. Is that right?
Yes, that is correct.
If you can't afford to do that, you can try to slide 1 1/2-inch-thick XPS insulation between the concrete and the studs. This job needs to be done in an airtight manner. It's going to be awkward -- much more difficult than spray foam. Every seam in the XPS needs to be carefully sealed with canned spray foam or high quality tape.
No, I cannot afford spray foam. I can only buy the 3/4 or 2" pink rigid foam board locally. If I want 1" or 1 1/2 " I have to order and I'm pressed for time.
After that work is done, I suppose that you can install rigid foam between the studs using the cut-and-cobble method if you want. I'm not a fan of cut-and-cobble, but you can do it. Again, spray foam would be easier. For more information on the cut-and-cobble method, see Cut-and-Cobble Insulation.
So, are u suggesting, foam behind the stud, then foam, bwtn stud, thus eliminating the need for fiberglass? Also, the 3/4 " is actually a sheathing (protective film on both sides) usually use for exterior purposes and not a board. Does this matter?
I might be going the gobble gobble method... Lol. I meant the cut and gobble. My son and I could easily do this.
So should I do the thin foam then fiberglass, or avoid fiberglass at all cost and just do rigid foam "pancake" style?
I'm in georgia, zone 3.
Thank you so much.
Response to Nat (Natasha) Hilton
Natasha,
Q. "Are you suggesting, foam behind the studs, then foam between studs, thus eliminating the need for fiberglass?"
A. No. I am suggesting closed-cell spray polyurethane foam, but you don't want to do what I suggest. When it comes to fiberglass, I thought that my article -- the article that is on the same page that you are now posting a comment on -- was clear on that point. I wrote: "Can I insulate on the interior with fiberglass batts, mineral wool batts, or cellulose? No ... I don’t think that fiberglass batts belong in a basement. My advice: if you want a higher R-value, just install thicker rigid foam, and leave the stud bays empty."
Ok, thank you very much.
Ok, thank you very much. Apologize for the confusion. I will be putting in a rigid foam behind the studs, then do the cut and gobble behind (2") rigid foam bwtn the studs. I will be sealing with great stuff gap and crack . Then sealing with a tyvek tape bwtn seams. No fiberglass!!;)
My ceiling already had fiberglass insulation. What are your suggestions on that?
The rotted rim joist is just below front stairs. Looks like at one time prev owners did some work on front stairs. Funny thing is my inspector suggested before purchase for prev owner to seal around stairs. One would think he would have gone in basement and check to see if there were any water damage. I've only been in the house 3 months and this was old rot. Well we had a good bit of rain last night and today, and I had someone to come check it out. Yep, water is seeping in. So, they will be replacing plywood with pressure treated plywood...etc..
Anyway, your forum was a God sent... Many thanks..
(Back to the basement now)
Roxul sound insulation
I'm thinking off putting in roxul sound batts in interior wall to help with noise control, and possibly some resilient channel on ceiling to help with noise control. Any thoughts on that?
And I believe I just ask another question on basement flooring? Would tiling then area rug be the best option?
Response to Nat (Natasha) Hilton
Natasha,
It's best to post comments that are relevant to the article where you are posting. Since your most recent questions aren't about basement wall insulation, you should post your questions on GBA's question and answer page. Here is the link:
https://www.greenbuildingadvisor.com/qa
Alrighty, thanks again..
Alrighty, thanks again..
Response to Fred Zheng
Fred,
Q. "I just received three quotes about finishing my basement. All three of them plan to insulate the wall with fiberglass batts... When I told them about the idea of using rigid foam board, I was told that they have never done it."
A. Walk away from these contractors and keep looking until you find a contractor who understands building science principles.
Q. "Could you please explain why it's also not good enough by leaving a space between the fiberglass batts and the concrete wall?"
A. The concrete wall is cold. The indoor air in your basement is warm and humid. The fiberglass insulation is air-permeable, so it doesn't prevent warm interior air from contacting the cold concrete. The usual result of this type of insulation job is that (under the right conditions) condensation forms on the concrete wall, drips down to the floor, and form puddles near the bottom plate.
leave space between fiberglass insulation and concrete wall
I live in West Chester, PA. I just received three quotes about finishing my 1700 sq ft walk-out basement. All three of them plan to insulate the wall with fiberglass batts between the frames and leave a 1.5-3 inch space between the insulation and the poured concrete wall. One contractor will also install a little computer fan in that space to keep the air circulating. When I told them about the idea of using rigid foam board, I was told that they have never done it, it's unnecessary and too expensive. From what I read so far, I understood that fiberglass batt should not be installed directly against the concrete wall. Could you please explain why it's also not good enough by leaving a space between the fiberglass batts and the concrete wall? Forgot to mention, I painted the concrete wall with dry-lock 2 yrs ago so my kids have a cleaner basement to play before I have it finished. Does the dry-lock paint provide some benefit or actually will cause problem now when to finish the basement? Thanks.
Thanks. A further complication regarding dry-lock paint
Forgot to mention, I painted the concrete wall with dry-lock 2 yrs ago so my kids have a cleaner basement to play before I have it finished. Does the dry-lock paint provide some benefit or actually will cause problem now when to finish the basement? such as using rigid foam against the paint? The curious thing is that one contractor wants to put two-coats of dry-lock on the wall and one coat of oil-based dry-lock on the concrete slab floor. Thanks.
Response to Fred Zheng
Fred,
Q. "Does the DryLok paint provide some benefit?"
A. Yes. It can limit moisture flow through the concrete.
Q. "Will the DryLok cause problems now when to finish the basement?"
A. No.
For more information on DryLok, see Fixing a Wet Basement.
Basement Wall Insulation
All I can say is, "wow" to all the articles and information on this site. I was a drywall contractor for the last 10 years in Ontario, Canada. I thought I knew a little about insulation. This blog has got me wondering.
The way basement walls are done up here (and nearly 90% of houses up here have them); 2 X 4 framed wall with tar paper backing (stapled to back of studs), no space between the back of studs and block wall, R12 fiberglass (blown or batt), then sealed with 6 mill poly and acoustic caulk. This is minimum code.
Everything I read on here is suggesting this is not a good process? Am I reading all this right?
Response to Shawn Currie
Shawn,
Q. "Everything I read on here is suggesting this method [fiberglass insulation with interior polyethylene] is not a good process. Am I reading all this right?"
A. Yes, you are reading this right. Building codes in much of Canada (and in many areas of the U.S.) are about 20 years behind the times when it comes to building science knowledge. Remember, just because a house meets code, doesn't mean it's well built.
The reason I stumbled upon
The reason I stumbled upon this website is because I started working for an insulation company. I wanted to know the building science behind the R Value myths, dew points and thermal bridging to help me educate potential clients.
Everything on here contradicts what we do. Right up to using polyethylene on the interior of a building when we are using rigid foam board on the exterior, osb and fibeglass in the cavity. Most contractors know it's wrong, but inspectors tell them it has to be done.
Here's my next question; if we have studies to show the effects of improper building practice, why is it taking 20 years to put them into place?
This is all very new to me, so forgive my ignorance.
Response to Shawn Currie
Shawn,
Local jurisdictions usually adopt a version of a model code, like one of the codes developed by the International Code Council.
The process to introduce an amendment to one of these codes is cumbersome, taking a minimum of three years. Lobbyists have to support these changes. That's expensive, and such work is usually supported by a group with a financial interest. It may take 2 or 3 code cycles (6 or 9 years) for a proposed amendment to be adopted.
After an amendment is approved, the new code is only a piece of paper until it is adopted by a local government. If adopted, it is often first amended. This process can take 2 or 20 years.
A few links:
http://mwalliance.org/sites/default/files/uploads/MEEA_2010_Midwest%20Energy%20Codes%20Needs%20Analysis%20Report.pdf
http://www.iccsafe.org/gr/Documents/AdoptionToolkit/toolkit.pdf
http://www.aamanet.org/upload/file/Codes_GM0512.pdf
http://pffacts.blogspot.com/2013/09/home-builders-slow-dysfunctional.html
The R-20 recommendation
Martin, I think I've found a flaw with your mention of the Canadian study that found R-20 to be most cost-effective.
Their R-20 was achieved using 5 1/2" batts. They compared it against XPS with R-10 and EPS with R-9 and both of those were more expensive to install and saved less energy than the batts.
I do agree with you that batts are a bad idea for moisture reasons, so the question that leaves us with is, how much foam is really most cost effective? Your article basically reads like this: "R-20 is most cost effective... foam is best... so, use R-20 foam". But the cost of R-20 foam may actually be an extravagance that doesn't make sense for a lot of people.
On the other hand, a DIY installation will have no labor cost, so thicker foam may have a chance to pay itself back within the 30 years used in that study.
Re-insulating an already finished basement
Hello Martin,
Thanks for this informative article. I'm just reading it today though it was posted two years ago.
We live in Wisconsin (zone 6) so winters are cold and summers can be humid--the dehumidifier runs all summer currently.
Some facts about the basement are:
1. Some will be playroom space, some will just be storage. Those areas are separated by doors.
2. We had all of the rim joist area spray foamed this summer all the way around the edge of the basement.
3. The existing finished space walls have a combination of furring strips and white styrofoam. There is also electrical with shallow boxes along the walls. We want to keep electrical in the space.
I have attached a photo of this.
4. Ceilings are 7-7'6", so not really deep for insulating the floor.
5. The basement is dry. We have a dehumidifier that still runs, so hopefully if we do something it will help with that (saving money for electric bill).
My questions are these:
1. Is it better to insulate some of the wall, like the two foundation walls of the finished playroom space (the other two are interior stud walls) if I can't do all of it all the way around the basement, or is it an all/nothing proposition in terms of humidity, insulation, etc.?
2. I want to put carpet on part of the floor of the playroom area. Any advice on that?
3. How would I seal the gap between the spray foam in the rim joist and any rigid foam I put on the wall (I did not think to put up the pink foam before having the rim sprayed. Would it work to run a bead of Great Stuff along the top of the pink foam?
4. How thick should the interior rigid foam be for zone 6 code -- 1"? 2"?
5. Can I adhere the foam to the concrete with Great Stuff or foam adhesive if I don't want to hammer drill it? Would it work if it wasn't adhered at all and the studs kept it in place?
6. Your advice is to leave stud cavities empty if there is rigid foam behind them, right?
7. What if I end up just putting drywall back up (it was paneling), do nothing with rigid foam and call it good enough if it's cost/time prohibitive to put foam in based on your advice? Our budget is small after foaming the basement and doing some other reinsulating through a contractor.
Thanks for any advice.
Corey Hansen
Response to Corey Hansen
Corey,
Q. "Is it better to insulate some of the wall, like the two foundation walls of the finished playroom space (the other two are interior stud walls) if I can't do all of it all the way around the basement?"
A. Yes.
Q. "Or is it an all/nothing proposition in terms of humidity, insulation, etc.?"
A. No.
Q. "I want to put carpet on part of the floor of the playroom area. Any advice on that?"
A. Carpet cannot be installed on a basement slab unless there is a layer of rigid foam under or over the slab -- especially in a basement that is so humid that it requires a dehumidifier every summer. In the summer, humid air will condense on the cold slab under the carpet, leading to mold. Either install tile flooring or vinyl flooring, or install a layer of rigid foam, followed by OSB or plywood subflooring and carpet.
Q. "How would I seal the gap between the spray foam in the rim joist and any rigid foam I put on the wall (I did not think to put up the pink foam before having the rim sprayed. Would it work to run a bead of Great Stuff along the top of the pink foam?"
A. Yes, canned spray foam will work there.
Q. "How thick should the interior rigid foam be for zone 6 code -- 1 inch? 2 inches?"
A. According to the 2009 IRC, a basement wall needs at least R-10 insulation when foam is used in Zone 6. That means 2 inches of XPS or 2.5 inches of EPS.
Q. "Can I adhere the foam to the concrete with Great Stuff or foam adhesive if I don't want to hammer drill it?"
A. Yes. Use a foam-compatible adhesive or an adequate amount of canned spray foam.
Q. "Would it work if it wasn't adhered at all and the studs kept it in place?"
A. Yes, although it's important in that case to tape the foam seams to make sure that the installation is as airtight as possible.
Q. "Your advice is to leave stud cavities empty if there is rigid foam behind them, right?"
A. Yes.
Q. "What if I end up just putting drywall back up (it was paneling), do nothing with rigid foam and call it good enough if it's cost/time prohibitive to put foam in based on your advice?"
A. You will live to regret your decision. Doing this work will never be easier than it is now.
Exterior Basement Rigid Foam Attached Garage Detail
I would like to insulate my new construction house including the basement with 4" of XPS rigid foam on the exterior side. How would I handle the attached garage in this scenario? Would the basement foundation be poured, then insulated on exterior, then garage slab poured?
ADA for basement stud wall?
Is it necessary to install the drywall in the airtight approach, when going over a stud wall (with batts) that has rigid foam behind it, up against the concrete? Image #2 in this article is the type of basement wall assembly I'm working with. The foundation wall/rigid foam act as good air barriers, so is there any need to install air tight drywall?
Response to Mark Fredericks
Q. "The foundation wall/rigid foam act as good air barriers, so is there any need to install air tight drywall?"
A. No, as long as the seams between the rigid foam sheets (and the perimeter of the rigid foam) have been properly air-sealed with high-quality tape, canned spray foam, or caulk.
Do the batts become less effective?
Thanks Martin, I'm working with foam that is sealed and pretty tight so I'm not worried about interior air reaching the concrete. I was curious if the effectiveness of the batt insulation would be reduced by allowing interior air to flow into the batts around electric boxes or through un-taped drywall seams. (I should have mentioned that I might not be taping the drywall immediately)
3" reclaimed fiber Polyiso faced in a Zone 5, confusion reigns
So Ive been waiting until Spring starts and the roofing contractors start ripping out old roofs and start to sell the reclaimed Poly on Craigslist.
Ive been planning to do this for the past 18 mths but want to pull the trigger now as the basement finishing now looks like a go (man cave finally)
Anyway Ive been speaking with Insulation Depot and they can do 3" 4x8 poly fiber faced for $21 a sheet, but the more Ive searched through GBA the more confused Ive become for two reasons.
The exterior of my basement is sealed with rubber membrane so water wont be an issue but the vapor internally on the cold concrete in winter is my concern.
So the plan was to glue it directly to the walls, tape sealing the seams and foam the bottom then put up a 2x4 stud wall, giving me an airproof seal
But then with the vapor going through the fiber/board, meeting the cold winter wall, encouraging condensation and mold, Ive seen on various sites, and the same people on those boards saying Poly but it had to be foil faced or XPS.
Ok so foil faced, is that the way I have to go, and then there is the "which way does the foil face" question Im scratching my head over as well.
So let me know 1. can I use the fiber poly on the basement wall or does it have to be foil faced and 2. if Im going foil poly which was does the foil face, to the wall or to the room?
Thank you
Response to Darren Finch
Darren,
1. Don't worry about vapor diffusion; just worry about airtightness.
2. Either kind of polyiso will work.
3. If you decide to use foil-faced polyiso, the foil can face any direction you want -- but if you face it toward the interior, and leave the stud bays uninsulated, the foil facing + air space will give you a little boost in the wall's R-value.
Girder pocket/basement foam
I am considering insulating the basement of my upcoming northern VT project with foam on the interior and have a concern. (or three). I was going to drop the main girder into pockets in the concrete wall at each end to be able to run the floor I-joists continuously above it but this seems to be a spot where air could leak through and condense on the cold concrete and damage the girder over time. Any thoughts?
Response to Caleb Creaven
Caleb,
If it's not too late, it's best if you don't install your beam in a beam pocket. With interior insulation, the beam pocket will stay cold and damp, and these conditions can eventually lead to rot at the end of the beam.
Beam pockets work best on foundations with exterior insulation.
If you plan to install interior insulation, you should support your beam on posts (columns) that bear on concrete footings.
Nope, not quite too late!
I would rather put the foam on the outside to keep the foundation warm and avoid frosty, wet concrete but I want to put 3 inches of EPS on and I don't love the flashing details necessary to cover the top. I'm doing a double-stud wall, albeit with some trepidation after reading the latest updates, so I don't want to cantilever the deck out over the foam. Anyway, decisions, decisions... thanks for your feedback.
EPS or XPS damp basement
I have quite enjoyed the Building Science articles about insulating old basements. I live in Zone 2 Saskatchewan, 1940s concrete basement with high seasonal water table (sump pump) and some wall weeping. A few years ago I insulated part of my basements walls with 2" XPS as recommended and am now in a position to do the remainder plus under a new partial slab. In light of the News story #2 in this article and the skyrocketing price of XPS, and articles like the 2013 one on thermal insulation below grade on Construction Specifier, would EPS not be an adequate choice? I have also read that EPS is the go-to rigid foam in Europe, because it is "greener". Thoughts? Thanks.
Response to Celeste Schmartin
Celeste,
Indeed, most green builders prefer EPS to XPS, because EPS has a benign blowing agent, while XPS has a blowing agent with a high global warming potential.
Walkout basement interior insulation
Thanks for this great blog! VERY informative!
I live in a split level home with a walkout basement in Minnesota. The house is built into a slope, so the basement wall facing the backyard is completely above grade. The rest of basement walls are half cinder block. I plan on using XPS against the block, then 2x4 with drywall. My plan was to use batting in the 2x4 cavities in order to achieve greater warmth in the winter (whoever installed the duct work did a poor job), but I'm discouraged from doing that after reading your article. Would I benefit by layering XPS board? For example, if I used multiple layers of XPS board against the block to achieve a 4 + inch layer of XPS, would I achieve a higher R value? Are there any negative of doing this, such as allowing moisture to build between layers of XPS?
Also, the home is 20 years old and there are 2x4 walls that sit on top of the block. The block is slightly jutted out on the interior, creating an approximate 2 inch "shelf." As far as I can tell, there is a thin piece of flimsy white foam (approx. 1/8" thick) between the top of the block and the wooden sill on which the 2x4 framing sits. There does not appear to be any mold issues on the sill, but the block is damp. I plan on putting XPS against the interior side of the block and on the top part of the block that juts out, butting the XPS up to the wooden sill. I am wondering how I should seal the joint where the top piece of XPS and the wooden sill meet. I will be unable to replace the sill without disassembling the whole house. Also, I'm pretty sure I won't be able to wedge any type of vapor barrier or insulation between the top of the block and the wooden sill.
I attached a picture so you can see what I'm dealing with.
Response to Joshua Fahey
Joshua,
Q. "Would I benefit by layering XPS board? For example, if I used multiple layers of XPS board against the block to achieve a 4 + inch layer of XPS, would I achieve a higher R value?"
A. Yes, you can install two or more layers of XPS. For example, 4 inches of XPS has twice the R-value of 2 inches of XPS.
Q. "Are there any negatives of doing this, such as allowing moisture to build between layers of XPS?"
A. No. There are no negatives. Thicker foam is better than thinner foam.
Q. "As far as I can tell, there is a thin piece of flimsy white foam (approx. 1/8" thick) between the top of the block and the wooden sill on which the 2x4 framing sits."
A. The thin white foam is called "sill seal."
Q. "I plan on putting XPS against the interior side of the block and on the top part of the block that juts out, butting the XPS up to the wooden sill. I am wondering how I should seal the joint where the top piece of XPS and the wooden sill meet."
A. Caulk will work just fine.
Insulating irregular walls and around a sewer pipe
Martin,
I am impressed that you've continued to comment on this thread and provide valuable feedback to the community.
I came across this post in researching how to insulate around my basement after tearing out an existing wall assembly that was riddled with mold on the backside of the wall and in the insulation.
My challenge now is that the wall is stepped with a sloped transition ledge where the two block sizes meet and a large cast iron sewage pipe spanning 3/5ths of the walls in the basement. In some cases, I will have no space or less than 1/4" space for insulation (e.g. around the collars of the pipe).
Additionally, I will not be able to get adequate insulation behind the water heater and build a wall assembly in front of the insulation.
The original wall assembly around the pipe was built 8" off of the concrete wall with paper-faced fiberglass insulation installed in the stud bays. Of all of the walls that I tore down, this is the only wall that did not experience discolored insulation and mold on the backside of the drywall.
I've since had an interior french drain installed with a dimple mat extending 4" up the wall.
I have been considering -- that given the circumstances of this wall and pipe -- it would perhaps be a better idea to get a thicker, continuous layer of rigid insulation installed against the exterior-facing side of the frame assembly glued and taped and leave an air gap between the wall assembly and the concrete. If I install it against the backside of the framing, I could use full, uncut sheets and minimize the number of joints and this would effectively prevent conditioned, moist air from moving through the wall assembly into the air gap.
So working our way in, it would be:
- Concrete block
- Air gap
- 2" XPS foam board, glued and taped
- Steel or wood studs; empty bays
- Mold resistant drywall
While I understand the air gap to be sub-optimal as compared to having the XPS against the blocks, does it seem that this is a better choice in this case given the irregular structure of the wall and the cast iron pipe? This would seem to also be the best option around the mechanicals where I would not be able to build a full wall assembly behind the water heater and air handler so instead, build around it 2" XPS directly attached to the wall assembly.
The contractor that installed the French drain also recommended laying PE VB directly against the concrete, tucked into the dimple mat at the floor. I think that this would be one way of keeping vapor out of the building entirely. The above grade portion of the foundation is anywhere from 8"-24" around the perimeter of the house so I think that there would be adequate surface area to allow the concrete to dry outwards.
I would love to hear some feedback on the above.
Response to Charles Chen
Charles,
First of all, pipes and water heaters are not like mountains, established by God and unmovable. Pipes and water heaters can be moved. If you aren't a plumber, you can hire a plumber to move your pipe and your water heater away from the wall.
That said, it may be that you don't want to incur the expense required to move the pipe and the water heater.
Your plan will work, but I urge you to pay close attention to airtightness at every step of the way. Since you will end up with a cold masonry wall, any moisture that reaches that will will condense.
insulating from the interior
Martin,
If we insulate from the interior using an EPS system with built in studs is no fire blocking required? or must there be fire blocking at the top?
Further, if XPS is put up against the wall to create the thermal break followed the 2x4 wall, must there be a fire break on top of the XPS? I'm assuming using a thicker 2x would accomplish this.
If there are any easier ways to meet the fire code on the XPS/EPS it would be much appreciated but I'm unclear on how the fire break code applies to foam area's as most of the code seems to refer combustible walls and/or around 2x4's.
Thank you!
Response to Michael Lutkenhouse
Michael,
My understanding is that fire blocking requirements apply to air pathways or stud cavities -- so if your wall has no air pathways or stud cavities, I don't think that the requirements apply. However, I am not a code official. The only interpretation that matters (in your case) is the interpretation of your local code official -- so call up your local building office and ask.
The article includes an image (Image #5, reproduced below) showing a location where fire blocking is required. For more information, see this article: Fire-Blocking Basics.
.
heavily insulating basement wall
Our basement has concrete walls and is uninsulated. We would like to add a foot of insulation on the interior.
I understand that mineral wool should not be in direct contact with the wall.
The idea is to insulate with:
1)3 inches (two layers, 1.5 inch each) of expanded polystyrene in contact with the concrete
2)9 inches of mineral wool built out with wood 2x4s and furring strips, covering the foam to the interior
3)plywood or cement board finish
It seems the mineral wool, instead of foam, would allow the wood to dry to the interior if they become wet.
Sharon- what climate zone ?
You're putting R12-ish foam on the exterior side of R36-ish rock wool, for a total R of R-48-ish, or about 25% of the total R being air-impermeable foam.
If you're on the cool edge of US climate zone 5 or colder a ratio of 25% is not sufficient for dew point control on the above-grade portion of the walls, and it would be prone to wintertime moisture accumulation (even frost formation) in rock wool on the upper part of the wall.
The interior side sheathing also be tight to the rock wool, with no intermediate air gaps/channels. If it's plywood it doesn't need to be painted, but if fiber cement it eitehr needs to have latex paint to bring it's vapor permeance into Class-III vapor retardency territory, or a smart vapor retarder between the fiber-cement and the rock wool.
thanks Dana
Thank you Dana.
Just to be clear, this would all be interior insulation.
I thought I understood the dew point risk when there is a mix of exterior and interior insulation.
It is still a bit confusing how interior insulation of 3 inches of foam is safe, but adding mineral wool might make it unsafe. It seems the air which traveled through the mineral wool would be closer in temperature to the air of the foam, and thus less likely to form frost, than if there were no mineral wool.
Response to Sharon Secrist
Sharon,
Q. "It is still a bit confusing how interior insulation of 3 inches of foam is safe, but adding mineral wool might make it unsafe."
A. Without any mineral wool, the interior face of the rigid foam will be at the temperature of the interior air (or close to it). In winter, the concrete wall will be cold, while the interior air will be warm. If the interior face of the rigid foam is warm, you won't get condensation on the foam.
If you add a thick layer of mineral wool on the interior side of the rigid foam, then the mineral wool insulates the rigid foam -- separating the rigid foam from the warm indoor air. The mineral wool keeps the foam cooler. With enough mineral wool, it's possible to cool the rigid foam below the dew point. Once you do that, you can get condensation on the interior face of the rigid foam.
Roxul vs Rigid Foam
I'm getting so many conflicting views and suggestions with how to insulate my basement. I don't have water issues, but I do get some moisture down there (who doesn't). When i moved in I installed a Sante Fe dehumidifer that keeps the whole space under 40% humidity.
I have one exterior wall and section of my basement that will stay unfinished (mechanical area) and then one adjacent wall that will be left unfinished the full length so workers can get to my electrical panel, plumbing pipes and main line etc (about 3 feet wide from the interior framed wall. These 2 exterior walls i'm recommended to have R15 Roxul comfortabatts pinned directly to the concrete and left as is without any type of barriers or anything. For the interior wall that will be up against the conditioned space i'm told to use Roxul safe and sound. Is this suitable for the unfinished space that won't have any drywall? This area will not be cooled or heated, but is where my dehumidifier is located (it will also be duct into the the finished area). Is this acceptable or should I use Rigid Foam board just left as is in the unfinished sections?
The other 2 exterior walls will be up against the interior conditioned space is. This is where I get into so many conflicting views..
One guy suggests that I use the same 3" roxul (R15) AFB and pin it against the concrete (not the roxul comfortboards, then have my contractor frame the walls, (no insulation in between the frame sections) then just drywall and a primer that will act as the vapor barrier. This doesn't seem to be advised at all here, is this proposed idea acceptable to avoid moisture and mold issues? Or should I use rigid foam against the concrete, then have the frame built, then either add roxul insulation or left empty, then drywall with primer and paint?
I'm in NY so I think R15 is more then enough for the finished space. Please help! - Appreciate it!
A bit confused...
Hi guys,
I’m starting the transition of my “crawl” space into living space. My home is fairly new construction and the lot is sloped from back (higher) to front (lower). The portion of my crawl that I want to convert to usable space is roughly 8’2 from the 4 inch slab I had poured (got ahead of myself doing that as I only have crushed rock + poly + 4 inches of concrete and didn’t think to put down foam before the pour).
My foundation walls (8 inches thick) range in height from 18 inches to 48 inches high with 2x6 stud (pony walls ranging from about 48 inches high to 79 inches high) on the foundation wall to the joists of the first floor. I am planning to frame full height walls inset from the concrete walls by a 3-4 inches.
I am in Seattle (Marine Climate 4 I believe) and the exterior of the foundation has no insulation.
My questions:
1) Should I bother to insulate the cavities in the existing 2x6 pony walls? If so, are fiberglass batts ok in this above grade cavity?
2) Should I use rigid foam (EPS or XPS) to cover the 18-48 inch high exposed concrete foundation walls?
a. I assume the answer is ‘yes’ so do I insulate the small parts of the tops of the concrete foundation walls that are not covered by the sill plates (8 inch thick concrete with pt 2x6 in top of it)?
3) I was planning to insulate the interior 2x4 walls I want to frame. Is it a good idea? What is best to use? I had planned to use faced fiberglass batts then sheetrock to finish.
Thank you in advance for helping me do it right the first time!
Greg
Response to Greg Volland
Greg,
Q. "Should I bother to insulate the cavities in the existing 2x6 pony walls?"
A. Yes. Insulation is a code requirement.
Q. "If so, are fiberglass batts ok in this above grade cavity?"
A. Barely OK. Fiberglass batts are the worst-performing insulation material sold in the U.S.
Q. "Should I use rigid foam (EPS or XPS) to cover the 18-48 inch high exposed concrete foundation walls?"
A. That would work, as my article explains.
Q. "I assume the answer is ‘yes,’ so do I insulate the small parts of the tops of the concrete foundation walls that are not covered by the sill plates (8 inch thick concrete with pt 2x6 in top of it)?"
A. Yes. You will need to install horizontal pieces of rigid foam, at least as thick as the vertical rigid foam installed on the interior side of the walls, to cover these concrete shelves.
Q. "I was planning to insulate the interior 2x4 walls I want to frame. Is it a good idea? What is best to use? I had planned to use faced fiberglass batts, then sheetrock to finish."
A. Again, fiberglass batts are OK. If the framed 2x4 walls are on the interior side of concrete walls, make sure that there is an adequate thickness of rigid foam between the concrete walls and the 2x4 studs.
Reply to Martin Holladay
I was trying to replicate your plan posted here: https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/7-steps-energy-efficient-house-1-basement ---- I couldn't tell if I was to insulate both the cavity of the 2x6 pony wall AND the full height interior 2x4 walls that I will frame out.
If I do the closed-cell spray foam, do I still apply the rigid foam board first or just spray to achieve at least the needed R-15 value in my area?
I am looking at DIY spray foam kits. Have you ever heard anything about Foam It Green kits? https://www.sprayfoamkit.com/spray-foam-insulation/products.html?gclid=CLynj5OgnswCFQQpaQodezoKkQ
Thanks for sharing all your wisdom!
Greg
Response to Greg Volland
Greg,
As I explained in my article, I don't like to install fiberglass batts in basements. Some builders do, however. If you choose to install fiberglass batts in a basement wall, make sure that you have a layer of rigid foam between the studs and the concrete wall, and make sure that the concrete is dry.
Q. "I couldn't tell if I was to insulate both the cavity of the 2x6 pony wall AND the full height interior 2x4 walls that I will frame out."
A. It's up to you to decide the R-value of your insulation. In Climate 4C, the 2012 IRC requires a minimum of R-15 for basement walls (assuming that we are talking about continuous rigid foam or spray foam insulation -- the best approach). That's a minimum R-value; you can install more insulation than that if you want to. Building science principles dictate that a thicker layer of fluffy insulation (if you insist on including fluffy insulation) requires a thicker layer of rigid foam or spray foam insulation. When in doubt, remember that thicker rigid foam or spray foam insulation moves your wall in the direction of less risk, while thicker fluffy insulation moves your wall in the direction of more risk. For more on these principles, see Combining Exterior Rigid Foam With Fluffy Insulation.
Q. "If I do the closed-cell spray foam, do I still apply the rigid foam board first or just spray to achieve at least the needed R-15 value in my area?"
a. It's possible to use 100% closed-cell spray foam if you want, as my article explains. If you go this route, make sure to leave a gap of between 1 and 2 inches between the studs and the concrete, and make sure that the spray foam installer fills this gap with closed-cell spray foam.
Q. "Have you ever heard anything about Foam It Green kits?"
A. I'm not familiar with those kits, but I see no reason why you shouldn't use that brand if you want to. I don't think that the word "green" in this context has anything to do with the environment, however. Here's how the web site defines "green": it says, "Our Signature Green Formula (Green means go - no mistakes, no waste)."
Reply to Martin Holladay
Thanks Martin.
I read the link you posted about combining rigid and fluffy insulation and am confused. The article discusses rigid foam on the EXTERIOR. That is not my planned application. My home's exterior is already complete (OSB + black felt + cement siding) that the builder put on. I am looking to do all my basement/over-height crawl work inside that envelope. Does make any difference?
Also, can I apply closed cell spray foam directly onto rigid foam to get the R values I want and seal up the 'shelf' at the top of the concrete wall as you put it and the rim joist areas or is it 'better' to just end up with a thicker coat of closed cell spray foam (i.e. multiple passes as per the instructions) and not do the rigid at all?
Originally, I was planning to do something along the lines of the attached example that I found on your site.
Greg
Response to Greg Volland
Greg,
When you are combining an air-impermeable layer of foam insulation on the cold side of the assembly with an air-permeable layer of insulation on the warm side of the assembly, there is risk of moisture accumulation if the foam layer is too thin (or the fluffy layer is too thick).
That is the principle discussed in the article I linked to (Combining Exterior Rigid Foam With Fluffy Insulation). The physics is the same -- whether the foam layer is on the exterior side of wall insulation, or whether the foam layer is on the interior side of a cold concrete wall. That said, the concrete isn't at the outdoor air temperature -- at least the bottom of the wall isn't. But the top of the wall may be.
I could discuss the similarities and differences between these different types of wall assemblies if needed, but such a discussion is probably unnecessary -- especially if you want to use just one type of insulation (closed-cell spray foam).
You have many options. It's up to you to choose on option.
100% closed-cell spray foam works.
100% rigid foam works.
A combination of rigid foam and closed-cell spray foam works.
A combination of rigid foam and fiberglass batts works -- as long as the rigid foam is thick enough, and the fiberglass is thin enough. If you want to go this route, tell us what R-values you have in mind for the various layers, and I'll give you a thumbs up or thumbs down.
Reply to Martin Holladay
Thank you again Martin!
Note: I have already insulated the 2x6 wall cavities that are on top of the foundation wall with R-19 faced fiberglass batts (I did this before I found your website!)
Here is my plan:
Option 1)
1. Adhere 1 inch R-Tech (R value 3.85) or 1 ½ (R value 5.78) EPS Rigid Foam to the interior of the concrete foundation walls (ranging in height from 18 to 48 inches – see pic).
2. Spray 2 inch coat of closed cell spray foam at top of EPS board to cover exposed top portion of concrete foundation wall.
3. Spray interior side of EPS board with 1 inch layer of closed cell spray foam (per Foam it Green, 1 inch thick = R-7).
4. Use rigid EPS + Foam combination to seal rim joists.
5. Build full-height 2x4 walls approximately 2 inches off existing foundation & pony walls, using PT bottom plate and sill seal.
6. If needed: Insulate 2x4 walls with fiberglass roll/batts (thinking R-13 / Faced)
Option 2)
1. Spray 3 inch coat of closed cell spray foam to cover all exposed concrete foundation walls, including the exposed top portion of concrete foundation wall as well as rim joist cavities.
2. Build full-height 2x4 walls approximately 2 inches off existing foundation & pony walls, using PT bottom plate and sill seal.
3. If needed: Insulate 2x4 walls with fiberglass roll/batts (thinking R-13 / Faced)
Thoughts?
Greg
Go rigid, not spray
At 3" closed cell foam would have to be applied in two lifts, with a curing period between them.
I'm not sure what the inch of foam sprayed onto EPS foam is buying you? (in option 1, part 3)
An inch of EPS with an R13 studwall meets code-min performance.
If you furred out the pony wall stud edges to make them flush with the foundation and installed cheap fluff in those furred-out stud bays (it compressed low density R30 or whatever, as long as it's full), then installed an inch of EPS floor to ceiling, seams taped, edges sealed, you can then install a 2x4 studwall on the interior of that.
Put an inch of EPS under the bottom plate of the stud as a capillary break & dew point control, no need for pressure treated. The pony wall portion would already be well over code min, but you'd still need to stuff the studwall with fiber where the concrete starts (but full height is also fine). Unfaced R15s would with margin, and would still be able to dry out after a flood. If the basement is always dry and flood risk low,kraft faced R13s would be fine.
If you left the pony walls empty you'd need at least R13 + 5 to meet code on your interior studwall which means 1.5" EPS instead of 1".
Response to Greg Volland
Greg,
Q. "I'm also getting a bit confused as to why some commentators at GBA are saying SPF is great and now you're advocating for no SPF and rigid only. Please enlighten me."
A. You'll hear many voices (and many opinions) on GBA. Here's my opinion: spray foam insulation is good for certain applications, but since most brands of closed-cell spray foam are expensive and use a blowing agent with a high global warming potential, it's best to use other insulation materials if these other materials work well. Save the spray foam for applications where it's the only option (like insulating a rubble-stone foundation wall, or certain air sealing tasks).
Reply to Dana
My thinking about using the closed cell spray foam over EPS is cost and time savings. The reason I was leaning towards some level of SPF is that it would help seal the space and irregular cavities/gaps and would be faster than EPS + caulking.
You kind of lost me with your suggestions... As my photo shows, I have faced R-19 in the pony walls already. Are you saying that if I furred out those walls to make them flush with the concrete foundation wall (about an inch), I'd have to put "fluff" in that small cavity between the EPS and the R-19? Not sure how I'd even do that..
I'm open to that idea of doing EPS floor to ceiling as you suggest and then the 2x4 walls inset. If I did that, would I need to insulate the interior 2x4 wall?
I'm also getting a bit confused as to why some commentators at GBA are saying SPF is great and now you're advocating for no SPF and rigid only. Please enlighten me.
Greg
Reply to Martin Holladay
I understand. What about the other part of my reply to Dana:
"As my photo shows, I have faced R-19 in the pony walls already. Are you saying that if I furred out those walls to make them flush with the concrete foundation wall (about an inch), I'd have to put "fluff" in that small cavity between the EPS and the R-19? Not sure how I'd even do that.
I'm open to that idea of doing EPS floor to ceiling as you suggest and then the 2x4 walls inset. If I did that, would I need to insulate the interior 2x4 wall?"
Thanks,
Greg
Response to Greg Volland
Greg,
Most of these decisions are up to you, not me or Dana.
If you want to make the interior finish material (probably drywall) of the lower concrete wall and the upper framed wall co-planar, you can. Or, if you prefer to have a shelf dividing the two walls, you can have that instead.
If you want to install EPS on the interior side of the insulated stud wall, you can. Or you can omit it -- as long as you meet minimum code requirements. The final R-value of these wall assemblies is up to you.
Reply to Martin Holladay
Martin:
I am planning to finish the inset 2x4 stud wall with drywall and want a continuous wall depth (no shelf).
What I am trying to understand is how best to insulate the exposed concrete foundation wall to avoid moisture/condensation issues. I have little concern for flooding, etc.
My specific question is this: if I furred our the existing foundation pony walls to be flush with the concrete foundation walls, and then ran EPS from floor to ceiling as Dana suggested, is it ok to have a 1 inch "void" between the EPS rigid foam and the existing R-19 that is in the current 2x6 pony walls atop the foundation wall?
Greg
Response to Greg Volland re Foam It Green
Edited - original comment was based on misunderstanding the proposed usage of foam kits.
I've used spray foam kits to seal/insulate rim joist with great success. In almost every circumstance it is cost-prohibitive to use foam kits in a high-volume project. Regardless, be sure the installed foam is properly protected to ensure fire safety. Fully review the relevant code requirements and manufacturer specifications.
Apology to Greg Volland
Greg-
My cautions regarding spray foam were based on a misunderstanding of what you propose. Please disregard them. Martin and Dana have thus far refrained from stating the obvious that "some people who post don't know what they are talking about and should be ignored." Their restraint is commendable. Listen to them, not to me!
Response to Greg Volland
Greg,
Q. "Is it OK to have a 1 inch void between the EPS rigid foam and the existing R-19 that is in the current 2x6 pony walls atop the foundation wall?"
A. It would be best to fill this void with something -- either a continuous layer of 1-inch EPS, or compressed fiberglass (perhaps a half thickness of R-13 batt, compressed).
Response to Martin and Dana
Thank you again for you insights. After closer inspection of the foundation wall and existing 2x6 exterior wall, I am having second thoughts about try to fur out the difference and fill the void because there are several obstacles in the way. What if I use attached plan? It calls for me to adhere 1 or 1 1/2 EPS to the concrete only (face and top, sealing the joints). Then build my new 2x4 wall inset. Would it be ok to then use R-13 Faced fiberglass on that 2x4 wall (ease of install and accessible)?
I think Dana was saying this would be an ok plan. Can you confirm?
Thanks! Hope to get going this weekend!
Greg
Avoid the void!
As drawn you have no back-side air barrier to the 2x4 R13s from the top of the EPS on up, and nothing but air between the R13 and the facers of the R19s. This is a classic thermal bypass channel (and too often, a rodent-run).
How you deal with it depends on the depth of the ledge. If it's a fairly consistent inch, double layering the EPS and going floor to ceiling with the interior-side sheet would be good, since both layers can be taped air tight. If it's 1.5" or 2", same thing. If more than 2" cutting the ledge-top EPS at the edge and stacking low density batts, then going floor to ceiling with the 1" EPS, compressing the batts in place with the studwall framing would stuff the void with a mid-density more air retardent fiber layer, and allow you to detail the foam as an air barrier. If the ledge is too inconsistent in depth to fill with EPS and under 2" you can use split 19s, which are very compressible down to about an inch or less, but would still be a reasonable density at 2". But the key is to have no voids, which means lower infiltration, no unimpeded thermal bypass, and no unimpeded convective routes around insulating layers.
As drawn the plywood subfloor and floor EPS stops at the bottom plate of the studwall, and the bottom plate is in contact with the slab. It's better if the plywood & floor EPS extend all the way to the wall EPS, which puts a thermal and capillary break between the slab and all of the wood (including the bottom plate) and exposes more bottom plate for attaching the drywall & kick board trim.
Reply to Dana
Dana:
Two thoughts based on your reply:
1) There is a 3 inch drain line on the wall that runs the length of the space (see photo). I can move it out from the wall enough to put the EPS on the existing studs + foundation wall in a continuous floor to ceiling application, but I would still need to bring out the new 2x4 wall far enough from that to account for the drain line. Is that ok in your view? Any suggestions? Should I just skip insulation in the new 2x4 walls?
2) Regarding the floor/stud walls, a few of the stud walls are already in place and the bottom plates are already down (PT + sill sealer roll foam) so, unfortunately the EPS is not going under those areas... I will see the way you suggest is done for the other areas.
3) there are a few posts on concrete footings in the space and the plan is to build 2x4 walls around them to "box them in". My question: should the slab areas within those "boxes" also be covered by the EPS? I assume so...
Greg
Hello???
Where did everyone go?!
Response to Greg Volland
Greg,
1. You'll get more responses to your questions if you post them on GBA's Q&A page. Here is the link:
https://www.greenbuildingadvisor.com/qa
Lots of readers visit the Q&A page daily. Far fewer readers are likely to notice Comment #161 on a 4-year-old article. (In general, the comment section under our articles is used to comment on the article itself -- not so much to post a question about a renovation problem. But of course we try to help people out, wherever the questions are posted.)
2. You've posted 9 questions in a row here, and you've gotten detailed technical answers to all but the last posting. Sometimes donor fatigue sets in. But we'll do our best to help you.
3. In response to your latest questions: Yes, it's OK to move the drain pipe and the 2x4 wall. And yes, you should install EPS against the slab before you box in anything that you intend to box in.
Reply to Martin Holladay
Understood. Thanks Martin!
wet basement walls
We are about to address the basement in the renovation of an 1860's carriage house in mid-coast Maine (zone 6). The building sits almost directly on a ledge at the front, jutting out about 15' into the basement and hugs one side of the building then opens up to about 6' of headroom towards the back. The foundation walls are stone and mortar closer to the ledge and change to brick as the basement opens up. The floor is dirt and after a decent rain or good thaw we have water running freely from the ledge and through at least one area of the brick.
We intend to address the water as best we can with French drains (exterior) and gutters, but the ledge guarantees we won't be entirely successful. We discovered a drain set in the basement floor, and though no sump pump exists water does seem to flow toward the drain and I don't anticipate the problem of standing water, just moisture passing through. We only intend to use the basement for storage of things like crates and tools, but want it as dry as possible.
We've had a parade of people through to quote the job and almost to a man everyone said we should spread gravel then poly sheeting (anything from 6-20mil depending on quote) and pour a rat slab which would provide a place for water to be channeled under the slab to the existing drain. They then all recommended 2"-3" of cc spray foam on the walls.
It all seemed to make sense, but I thought I'd come back to GBA one more time to make sure we weren't missing anything. Your article seems to point to one flaw in this plan, it doesn't seem to address our wet walls. Is the application of a dimpled membrane the answer? I'm not turning up much in a search, but is something like Superseal what I should be looking for? Also, did I also noticed the mention of an air space between the membrane and the insulation? If so, what will the spray foam adhere to?
Thank you and by all means, let me know if you see anything else we're missing.
Kim
Resonse to Kim Dolce
Kim,
1. If I were you, I would want to know where the floor drain leads. Does it run to daylight? If not, where does it go? How will you prevent it from clogging in the future if you don't know where it terminates?
2. The standard solution in a case like yours is to install dimple mat on the wall before you spray the closed-cell spray foam. The dimple mat should extend down to the crushed stone layer under the rat slab so that any water that trickles down the wall will have free access to the crushed stone layer.
Resonse to Kim Dolce
Kim,
If you post your question in the Q&A section rather than in the comments of an article like this, I think you'll get more people seeing it and responding to it. Martin's comments are usually the most thorough and helpful--the rest of us just function like a Greek chorus to help emphasize what he says. But sometimes what we say is useful.
My suggestion is to specify Lapolla 4G spray foam, because the standard closed-cell spray foams us a blowing agent that has a huge global warming impact (~1000 times worse than CO2).
Martin,
We do plan to put a
Martin,
We do plan to put a hose in that drain to see if we can figure out where it goes. There's a retaining wall at the back of our property and my money has been on that. Funny, until this moment it hadn't dawned on me to examine the retaining wall for a gusher during a heavy rain. Thanks!
kim
Charlie,
Thanks for the tip.
Charlie,
Thanks for the tip. I usually spend my time on the articles and blogs so don't tend to think of the Q&A.
kim
XPS VS EPS - Making my purchase today to begin project
Before I finalize my decision and place order today I am hoping to get some reassurance / advice. I am finishing my 2500 sqft basement which has poured concrete walls. I originally was going to do 2" XPS on perimeter of wall (Pink Stuff) and 1" XPS foamular 150 on floor with T&G OSB or plywood sub floor on top. However when at HD, I saw a product called DuroSpan, also goes under the label DuroFoam in Canada, that is 2" EPS with foil like film on both sides which cost almost $10 less than XPS. I was considering using this product for the walls, however decided against it because I am concerned the foil like lining would permit any moisture from concrete wall from being absorbed and dry inward. Although XPS has a low permeability, my thought is at least if some minor leakage did occur from a hair line crack at least it could be absorbed and dry inward, otherwise, if / when there is a minor water leak where would the water actually be able to dry to? However now I have found 2" closed cell EPS non-faced R8 for only $10 / sheet which would be over $15 savings per 4x8 sheet, being I need over 70 sheets this is a significant savings. My thought is the un-faced EPS would allow moisture / water to be absorbed in the event of a minor leak and dry inward, plus save me over $1,000 on material. For the concrete floor I would still use 1" XPS (Pink stuff), and have found the Foamular 250 as opposed to the formula 150 for even greater compression strength for only a $1 to $2 per sheet more. Am I missing anything? Will the EPS perform as good or better in this installation? XPS would be R10, EPS is R8, however I will be framing and filling stud cavity with batt insulation anyways. I will attempt to link to the EPS I am considering here: https://www.menards.com/main/building-materials/insulation/insulation-panels/expanded-polystyrene-foam-insulation-2-x-4-x-8-r-8/p-1444435971902-c-5779.htm?tid=-1979961929525189151&ipos=1 - Thank you for any insight back. I appreciate it. (Also I live in Climate Zone 5 in South East Michigan, however I don't think that is relevant to my question.)
Response to User-6948667
User-6948667,
You have double-posted your question, which is confusing. It's best to post your question just once.
Here is a link to the other place where you posted your question; you'll find my answer there: Interior Basement Insulation - XPS vs EPS selection.
Broken link (Click here to see a climate zone map.) Read more:
The climate zone map link is broken.
Response to William Sherman
William,
Thanks for notifying us of the broken link. We appreciate it. The link has been fixed.
Exterior XPS w/ dimple mat?
I would prefer to insulate the exterior of my new home's basement wall. I also plan to use a spray on elastomeric waterproofing membrane and a dimple mat. After the membrane, would you recommend the XPS and then the dimple mat, or the other way around?
XPS or EPS
My basement does not get very much air flow. Which would you say is better for indoor air quality:
1) DOW XPS with BLUEDGE (non HBCD flame retardant) but diffusing blowing agent
2) EPS with HBCD flame retardant
Thanks in advance!
@ Bryanw511, for insulating
@ Bryanw511, for insulating foundation exteriors, I recommend type 2, 15 psi EPS treated with borates for insect resistance. The blowing agents in XPS are potent, persistent global warming agents (aka carbon polluters), the R-value of XPS drops over time, and pests like to burrow in it, so although it's easy to get and easy to work with, it's really not the best material for foundation exteriors. The dimple mat can go on either the interior or exterior of the insulation, since water won't affect the foam, the air gap is small enough that it won't affect the insulating value significantly, and the reason for the dimple mat is to keep water away from the concrete.
@User 7xx, the diffusing
@User 7xx, the diffusing blowing agents in XPS are not a health concern, as far as I know, they just destroy the atmosphere. I don't know enough about the health concerns of flame retardants to advise you, but would ask why you don't consider increasing your ventilation to healthy, comfortable rates?
85 year old walkout with joists set in concrete
An energy audit identified foundation insulation as my most important step. While I would prefer to dig and insulate the exterior, a slab-on-grade addition on one side of my house makes insulation from the exterior nearly impossible for that side. Other sides have complications too, so insulation from the interior seems to be my option.
I'm in Southeast Alaska, which is wet and has an average Dec-Feb low of 26 degrees F.
My 85 year old walkout basement has some efflorescence and dampness, but no bulk water problems that I'm aware of. The slab floor has been completely dry. During a 100 year flood event last fall a failed storm drain broke like a fire hydrant next to my foundation, and the only impact was some dampness and efflorescence on the cement foundation wall there. With french drains, sloping, and some bituthene, I'm addressing exterior drainage to hopefully limit what efflorescence and dampness I do see.
Roughly 5 years ago the prior owner painted a cement sealant onto the interior foundation wall. That suggests there had been at least dampness in the past. It also means drying to the inside is already limited.
I see a few cracks in the concrete foundation wall, but no water coming through them. The cracks are in areas that were sealed by the prior owner, and areas that were not sealed by the prior owner. Given that it's 85 year old concrete, I find a few, dry cracks to be not so bad.
Complicating matters, my joists are set in the concrete foundation wall. It appears they were dipped in tar first, to limit susceptibility to water from the concrete. While I see efflorescence around them, they are hard and appear not compromised despite being in concrete for 85 years. Still, they're my primary concern.
I plan to insulate from the inside with 2" EPS against the foundation, and 2" polyiso to the interior of that. That achieves R20, avoids polyiso's R loss in the cold, and avoids the need to frame a wall thanks to polyiso's smoke and flame rating. Both materials have fairly limited greenhouse gas implications.
I'll appreciate any advice, and have a few specific questions:
1) How concerned should I be about making any changes since my joists are in the cement? I may be making the cement wetter if it can't dry to the inside, but my hunch is that it's fairly wet already, can't dry much to the interior already, and my exterior drainage work is making it less wet, regardless of the interior insulation. Is there any benefit to not insulating the cement rim joist area? Is insulating my foundation at all out of the question?
2) Is there any benefit to insulating from the exterior where possible, and having overlap sections where both interior and exterior are insulated to avoid heat moving through the concrete there? Or should just I stick with the continuity of entirely interior insulation?
3) It's been suggested I install drainage mat against my interior foundation wall before rigid foam. But since I do not have an interior french drain or bulk water problems right now, I think that would just allow warm air to get behind the rigid foam and cause moisture problems while reducing the insulation value due to cold air flowing down the drainage mat.
4) Since I do see some cracks in the concrete wall, should I be concerned that insulating the foundation could cause it to occasionally freeze, and crack more significantly, even though it's slightly below grade at the cracks? Should I be insulating less than R20, as suggested for a brick building here: https://www.greenbuildingadvisor.com/blogs/dept/musings/insulating-old-brick-buildings?
Thanks much for any advice folks have. I enjoy and appreciate GBA tremendously! Despite my inability to figure out how to add my name, I'm a GBA subscriber and it's a worthwhile investment.
There is no point to using EPS on the exterior
There is no point to using EPS if the average winter low is +26F. Inch for inch even the worst polyiso out there will beat it's performance with the cold side at 26F, and below grade the temperatures are much warmer.
In the attached file take a look at Figure 2 and compare the the derating curve for R5.2/inch roofing polyiso, and compare that to the Mineral Wool Batt curve (which is comparable to EPS). When it's 25F on the cold side, 65F on the warm side, the mean temp through the foam will be about 45F.
At 45F a polyiso- only solutoin would be delivering R5/inch. If the outer half is mineral wool (or Type II EPS), the mean temp through the outer layer would be 35F, and at best breaking-even with the performance of polyiso.
Below grade it's warmer than that, and the polyiso wins every time, even on the outer layers.
Most foil faced polyiso would outperform the straw-man polyiso example in that picture, thus beating EPS by even more. (R6-R6.5/inch @ 75F mean temp rather than R5.2.)
Response to User-7062640 (Comment #178)
User-7062640,
First of all, can you tell us your name?
Q. "How concerned should I be about making any changes since my joists are in the concrete?"
A. This is a hard risk to evaluate. There are lots of variables. Briefly, it's important to insulate your foundation and rim joist area; and this work will make the concrete colder, increasing the risk that your joist ends will rot.
The classic solution is to cut off the embedded ends of the joists with a Sawzall and support the joists with a new framed wall on a secure footing.
Q. "Is there any benefit to insulating from the exterior where possible, and having overlap sections where both interior and exterior are insulated to avoid heat moving through the concrete there?"
A. Again, this is hard to evaluate. Ideally, either all of the insulation will be on either the interior or the exterior of the wall. But with embedded joists, an argument can be made in favor of installing as much exterior insulation as possible, even if the exterior insulation doesn't extend all the way down to the footing.
Q. "Should I be concerned that insulating the foundation could cause it to occasionally freeze, and crack more significantly, even though it's slightly below grade at the cracks?"
A. No. The problem you describe almost never occurs.
Thanks Dana and Martin
Thanks for your insight. Dana, that graph is super helpful. Martin, I really appreciate your answers as always.
If you have any thoughts about the drain mat, or rain slicker, between the insulation and foundation, please do let me know. I plan not to use drain mat, and to secure the foam board with anchors (not adhesive) so I can aways cut the spray foam around the edge and take a piece of polyiso off 5 years down the road to see how things look.
-Shawn Salias
Response to Shawn Salias
Shawn,
The interior dimple mat is optional. If you see no signs of water entry, and if there is no interior French drain, the typical approach would be to skip the dimple mat.
half basement with concrete sill
What is the best way to insulate my basement wall? (picture attached)
Response to Jared Weber
Jared,
Well, if he's still there, the easiest way would be to tell the spray foam guy in the photo to aim his wand at the walls.
.
Second response to Jared Weber
Jared,
I'm not sure there is a "best" way.
The above-grade portion (the portion with stud walls) is insulated the same way as any above-grade wall.
The concrete portion of the wall is insulated as described in the article on this page.
If you want the interior wall to be co-planar, you can install floor-to-ceiling studs on the interior side of the insulation once the insulation work is complete, and then install drywall or plaster.
half basement insulation
That handsome guy in the super-suit was painting the ceiling joists (which will be left exposed)... same guy will be doing the insulation...
Thanks for your input. I guess I have a couple additional questions.
First, is the standard pink fluffy stuff I see in Home Depot the way to go for the above grade portion? I also see there are alternatives at the store such as Rock Wool and various types of foam panels
Second, for the space between the ceiling joists, which is hard to see in the picture... I saw a video where XPS foam and "great stuff" was used to insulate the rim joist area -- my rim joist area isn't exactly the same as the video because it doesn't' but up right against the foundation wall -- what would you suggest for this area?
Lastly, and maybe this is overkill... but if I used traditional insulation between the studs would it be a good or bad idea to put 1/2" XPS over the studs before I install my drywall to get a little bit more R-value down there?
Third response to Jared Weber
Jared,
Now that I think about it, I should have realized that the guy in the suit is a painter, not a spray foam contractor, since his wand has only one hose, not two hoses.
Q. "First, is the standard pink fluffy stuff I see in Home Depot the way to go for the above grade portion?"
A. It sounds like this type of work is new to you. If possible, try to find an experienced contractor to guide you. Either fiberglass batts, mineral wool, cellulose, or spray foam can work, but whatever insulation material you use, it has to be installed properly. In case you end up using fiberglass batts, I'll provide a link to a relevant article: "Installing Fiberglass Right."
Q. "I saw a video where XPS foam and Great Stuff was used to insulate the rim joist area."
A. Here is a link to an article to advise you on that work: "Insulating rim joists."
Q. "If I used traditional insulation between the studs would it be a good or bad idea to put 1/2 inch XPS over the studs before I install my drywall to get a little bit more R-value down there?"
A. Installing interior rigid foam is fine, but most green builders avoid the use of XPS, which is manufactured with a blowing agent that has a high global warming potential. Either EPS or polyiso would be a better choice. Here are links to two articles you should read:
"Walls With Interior Rigid Foam"
"Choosing Rigid Foam"
Insulating over dimple mat
If I already have dimple mat installed with protruting ramset fasteners halfway up the interior wall (to grade level) and the top half of the interior wall is exposed concrete, does it make sense to do something like 2" of polyiso on the top half, 1 1/2" of polyiso on the bottom half, and then 1/2" polyiso (to get to R13 or greater on the full assembly) as a second layer over the entire wall to level everything out/cover the seams?
The rigid foam would be flush against all concrete surfaces, but I doubt I could fully airseal the bottom half of the wall between the platon mat and the rigid foam (potential condensation issues), and I'd also be concerned about the protruding ramset nails puncturing the back of the foil backing on the polyiso as I attempt to level everything out on the wall (potential moisture transfer through the nails direct into the polyiso foam, but it would still have the dimple mat between it and the concrete)
Would spray foam insulation be a better choice or am I OK with the rigid foam approach? For cost reasons I'd obviously prefer the rigid, but not if it will cost me more down the road. Thanks in advance.
Response to Jay Lista
Jay,
It looks like you have posted your question twice, on two different pages. I have already answered your question on the page where you first posted it.
Here is the link: "Foam over Platon dimple mat on foundation."
Hi Martin
I think my post was confusing - I'm insulating the basement on the inside, not the outside. I reposted here since this thread made more sense than the last one that i now see was discussing insulating on the outside with dimple mat. Sorry for the mixup.
Second response to Jay Lista
Jay,
Using rigid foam would work, in spite of the imperfections you mention. Closed-cell spray foam would be easier and would perform slightly better. Is spray foam worth the higher cost? You decide.
Interior +/- Exterior XPS
Thank you for your very helpful article - I am new to this and I have been learning a lot from you. We are building a house in Nova Scotia, Canada, Zone 6A. It is near the ocean and the weather is often wet. The lower level of the house is 20 x 50, one 20 foot wall is all below grade, one 50 foot wall is a step-down foundation and the 2 other walls are above grade (the hill slopes in 2 directions). The lower level will be finished and I very much want to minimize any moisture. The builder plans to install a double run of footing drains and I believe a surface drain is also planned. We will have a metal roof with 2 foot overhangs. The current plans for the basement part is to have a 10" concrete wall, 2 inches of XPS (one 2 inch layer, interlocking sheets, seam sealed etc.), and a 2x4 framed wall with rock wool batts in cavity. Our code requires a vapour barrier before the dry wall - I am hoping they will accept a smart barrier like Membrain as a substitute. My questions for you are:
1. Should we also add 2 inches of XPS on the exterior? Are there any issues with sandwiching the concrete between 2 non-breathable layers? Do you think any benefits would be worth the extra expense? If we go this route the builder wants to slightly cantilever the house over the foundation.
2. Should we consider increasing the interior XPS? Interior space is not an issue for the 20 foot wall so we could easily increase the thickness there. Space is more of an issue on the 50 foot with the step-down foundation but it wouldn't be impossible to add another inch.
3. Are there benefits to using 2 1-inch layers of XPS, rather than a single 2-inch layer (e.g., to offset the sheets, etc.).
4. Any advice on avoiding the vapour barrier? The builder understands my concerns but thinks the inspector will require one.
I'd appreciate any advice you have. Thanks very much - Janet Ingles
Response to Janet Ingles (Comment #192)
Janet,
Q. "Should we also add 2 inches of XPS on the exterior?"
A. In the U.S., codes require basement walls in Zone 6 to be insulated to R-15. Your current plan (2 inches of interior XPS, which equals R-10, plus 3.5 inches of mineral wool, which equals at least R-13) gives you an R-23 wall, which exceeds U.S. code requirements.
So the answer to your question depends partly on your R-value goal. If you add another R-10 to the exterior, you'll be up to R-33. That's not crazy, if you can afford it.
However, green builders avoid the use of XPS, which is manufactured with a blowing agent that has a high global warming potential. What you want is EPS, not XPS. More information here: "Choosing Rigid Foam."
Q. "Are there any issues with sandwiching the concrete between 2 non-breathable layers?"
A. No.
Q. "Do you think any benefits would be worth the extra expense?"
A. You'll have to do the math. The answer depends in part on the cost to install the foam and your local energy cost.
Q. "If we go this route the builder wants to slightly cantilever the house over the foundation."
A. That's a normal approach.
Q. "Should we consider increasing the interior XPS?"
A. I may sound like a broken record, but the answer depends on your goals and your budget. Are you building a Passivhaus? Can you afford the upcharge? Or are you looking for a quick 5-year payback on any incremental work? I can't answer these questions for you -- your values and budget affect the answer.
Q. "Are there benefits to using 2 1-inch layers of XPS, rather than a single 2-inch layer (e.g., to offset the sheets, etc.)."
A. Yes. The thermal performance of two layers with offset seams will be slightly better than the thermal performance of one layer. Is it worth the upcharge in labor? You decide.
Q. "Any advice on avoiding the vapor barrier?"
A. Pray for an intelligent, well-educated building inspector. Failing that, try gentle education. If that doesn't work, install MemBrain.
Interior +/- Exterior XPS Continued
Martin - Thanks so much for your helpful and prompt comments. Cost aside, how much impact would adding exterior foam and/or increasing interior foam have on basement moisture? One of my primary goals is to avoid the damp/musty smells that I associate with basements - Janet
A few comments for Janet Ingles
The R10 XPS eventually drops to R8.4 (the same as 2" EPS of the same density) as it loses it's climate damaging HFC blowing agents. The manufacturers will warranty it to 90% of the labeled R value as long as it's not in a location where it can become saturated wet. That would make it R9. So from a design point of view consider it to be R8.4-R9 above grade (where it can drain), and "who knows, way better than nothing", below grade. The better drainage you have on the backfill, the more likely it is to still perform between R8.4-R9 in 25 years. But using Type-II EPS would be R8.4 now, and R8.4 fifty years from now, which is fine.
An R13 studwall is not the same as R13 continuous- it's closer to R9- R10 "whole assembly" for that layer after accounting the thermal bridging of the studs. From a thermal performance point of view you're looking at about R20 "whole-wall", with credit given for the R-value of the gyproc & concrete, and the interior & exterior air films, the 2" foam, and any finishing materials over the exerior of the foam, etc.. That would meet code for US climate zone 6 (which is comparable to the climate of NS). The thermal mass of the concrete also gives some benefit, but not enough to really matter in your climate & stackup.
With the 2" of foam on the exterior you could put the vapor barrier between the studwall and concrete (rather than on the interior side) and still meet the requirements of the NBC. I'm not sure if NS changed any of that for local codes. See Table 9.25.5.2 on p.30 (PDF pagination):
https://www.nrc-cnrc.gc.ca/obj/doc/solutions-solutions/advisory-consultatifs/codes_centre-centre_codes/revisions_errata-revisions_errata/2010_nbc_revisions_errata_november2013_2nd_printing.pdf
All locations in NS have fewer than 5000 (base 19C) annual heating degree days (Halifax runs about 4200 HDD/year) so as long as the foam-R/cavity-R ratio is more than 0.2, the code enforcers should be happy with the vapor barrier on the concrete. With the foam derated to a fully depleted R8.4 and R15 rock wool in the stud wall there would be a ratio of 0.56 at center cavity, which is good enough to meet code even in a 9,999 HDD(C) climate. With the labeled R10/R15 it would be a ratio of 0.66. In the US the IRC code prescribes a minimum of R7.5 on the exterior of a 2x4 wall for zone 6, to be able to use standard interior latex paint as the interior side vapor retarder:
https://up.codes/viewer/wyoming/irc-2015/chapter/7/wall-covering#R702.7.1
Your stackup would meet/beat that too.
Putting the vapor barrier between the studwall & concrete provides a capillary break between the concrete and the fiber insulation & wood, both of which could otherwise wick ground moisture if the footing is seasonally wet.
See also:
https://www.greenbuildingadvisor.com/community/forum/building-code-questions/59781/poly-and-canadian-building-codes
extra foam insulation
The payback on extra foam will be much better above and near the surface. So consider adding exterior foam that doesn't extend all the way down to the footings.
Plan on doing some dehumidification to prevent musty smells.
Response to Janet Ingles (Comment #194)
Janet,
Q. "One of my primary goals is to avoid the damp/musty smells that I associate with basements."
A. The origin of any musty smells won't be your walls (if the walls are detailed as described). The origin will be your slab floor (if it isn't insulated). The solution -- the key to a basement that doesn't have the musty smell -- is to install a continuous layer of horizontal rigid foam under your slab.
Musty smell mitigation...
The musty basement smell can also be mitigated by insulating under the slab (all of the slab). The subsoil temperatures in NS are lower than the summertime outdoor dew points, so moisture tends to collect on anything resting on the cool slab, creating mold and "musty basement smell" conditions. An inch of EPS does a lot for the musty basement smell, but 2" can still be financially rational on a lifecycle basis if you energy costs are high or your average construction costs low.
It's fine to use Type-II (1.5lbs per cubic foot nominal density) or some somewhat less expensive 1.25lb density Type-VIII EPS under a residential slab, but not under the footings. Building materials reclaimers usually have it in stock, stripped from large commercial buildings during demolition or re-roofing. (They also tend to have roofing polyisocyanurate, which is higher R/inch, but not suitable for use under a slab due to it's moisture wicking characteristics.) Used roofing EPS is typically less than 1/3 the cost of new foam.
edited to add:
(Looks like Martin is posting while I'm typing, again... :-) )
Interior +/- Exterior XPS Continued++
Thank you both very much for your comments...We were planning 4" rigid foam underneath 4" concrete (under the entire slab with vapour barrier between). We want to avoid musty smells but also prevent too cold winter feet...
ps. should we be insulating under the footings too?
Response to Janet Ingles (Comment #199)
Janet,
Q. "Should we be insulating under the footings too?"
A. Most people don't. Energy fanatics, including the Passivhaus crowd, often do. That said, a capillary break between the footings and the basement walls is always a good idea. For more information on this issue, see these two articles:
"Foam Under Footings"
"Capillary Breaks Above Footings"
Interior +/- Exterior XPS Continued+++
One last question: Because of the 50' house length we will have a 10" concrete wall. Would 2" of exterior foam have less of an effect warming the concrete than if the foundation was slimmer? I'm imagining (perhaps naively) that 2" of foam would have a harder time increasing the interior temperature compared to a wall that was slimmer. If so, would we be better off taking that foam and adding it to the interior layer (i.e., going from 2" to 3-4" foam inside)? Thank you all very much for your comments. I'm definitely in the "a little bit of knowledge is a dangerous thing" category...Janet
Response to Janet Ingles (Comment #201)
Q. "Would 2 inches of exterior foam have less of an effect warming the concrete than if the foundation was slimmer?"
A. No. Each layer of the wall assembly adds to the wall assembly's R-value. Your wall assembly will have interior mineral wool with an R-value of R-13, interior XPS with an R-value of R-10, 10 inches of concrete with an R-value of about R-1.7, and exterior XPS (if you add it) with an R-value of 10.
The R-value of the wall assembly is 13 + 10 + 1.7 + 10 = 34.7
If the concrete wall were 8 inches thick instead of 10 inches thick, the concrete layer would have an R-value of R-1.35 instead of R-1.7, so the total R-value of the wall assembly would be very slightly less (R-34.35 instead of R-34.6).
In short, the R-value per inch of concrete is very low. But it does have R-value.
In both of the above cases -- the case with the 8-inch-thick concrete wall, and the case with the 10-inch-thick concrete wall -- the contribution of the exterior XPS is the same (R-10).
What are your thoughts on exterior foam insulation that is only on the below grade portion of the foundation wall? One of the main purposes would be to help with moisture control. If we run it above grade it protrudes out from the siding substantially and cantilevering the house isn't possible with the current plans. If we run it just below grade how do you prevent water from infiltrating behind it?
For foam that extends above grade to the top of the foundation, Z-flashing that running from behind the siding and weather resistant barrier to the exterior of the above grade foundation foam is the standard solution. It works fine even if the exterior face of the foam is proud of the siding.
With foam that's only below grade (not really a great idea) you don't worry about it. The foundation still needs to be waterproofed and drained properly, with the surface appropriately graded to slope away from the building. The foam itself isn't particularly waterproof at the seams or the at the bottom edge (or even in the middle) and would let ground moisture by in any event. Using dimple mat between the foam & foundation to allow bulk water to drain quickly to the footing drain is a tried & true approach, whether full height to the foundation sill, or stopping at grade.
The foundation still needs to be fully insulated to meet code, whether on the interior or exterior. Insulating the interior just at the above grade portion doesn't cut it, since the path from the exposed above-grade exterior to uninsulated portion of the interior has too low an R-value- it's a huge thermal bridge. In cold climates insulating the interior to just the 4' below grade helps, but it's nowhere near code-minimum performance.
I live in a 110 year old house in Wisconsin that has a "rubble" foundation made of sandstone block and lime mortar. I have been doing a lot of research, and the recommendations about how to properly maintain this type of foundation seem to be diametrically opposed to the recommendations on how to insulate a basement. My understanding is that this type of foundation is designed to let moisture wick through pretty much constantly, and that halting that flow will cause the mortar to break down rapidly.
The previous homeowners installed drain tile at the base of half the walls, then painted those walls with impermeable paint and covered them tightly with XPS. Those walls are clean and dry on the outside, but I pulled one down as part of another project and was concerned with what I saw. It is natural for the mortar to slough off over time, but it seems to be happening more rapidly on the covered walls than on the exposed ones. I'm also concerned that the mortar "sand" will eventually clog up the drain tiles.
It seems like I could just repoint the walls and seal them up with XPS again, or I could instead do something that allows the walls to "breathe" more. But that feels like making a choice between a potentially short term solution and a crumbling foundation, versus a long term solution with a damp musty basement.
Do you have any advice on how best to insulate this type of wall?
Chuck,
Q. "Do you have any advice on how best to insulate this type of wall?"
A. Yes. Read the article on this page. In my article, I wrote, "If your basement has stone-and-mortar walls, you can’t insulate the walls with rigid foam. The only type of insulation that makes sense for stone-and-mortar walls is closed-cell spray polyurethane foam."
Of course, all the usual caveats apply: Before you can insulate any type of wall, you have to address any water entry issues. To do this, you may need to change the exterior grade or improve the footing drains.
With an older stone-and-mortar wall, it may be necessary to remove loose mortar and add fresh mortar where necessary before insulating. You might also need to consult an engineer to make sure that the old foundation walls are structurally adequate.
If you've taken these steps, you can remove the XPS insulation -- which never should have been installed -- and you can insulate your walls with closed-cell spray polyurethane foam. Note that most building codes require any type of foam, including spray foam, to be protected by a so-called thermal barrier (gypsum drywall) for fire safety.
Closed-cell spray polyurethane foam is a vapor barrier, air barrier, and a barrier to liquid water. It also has structural qualities -- it will fill the cracks in your wall and strengthen it. All of these qualities of closed-cell spray foam make it appropriate for stone-and-mortar walls.
Hi, I have a 1950s home in Western Colorado (high desert) with a mostly finished basement that is partially insulated (all but a closet and a large game room) on the inside. I have no idea of the R rating (probably fiberglass so about R10-13ish) or quality of the work. Temperatures get chilly during the winter with floors and some walls in direct contact with the ground and/or outside air. I have been considering adding 2" of exterior insulation board to the walls that I can access. Due to sidewalks, garage, and windows with egress, this only amounts to about 25% of the walls or about 40% of the two "longer" walls. If done, I plan on doing the digging and placement of the board myself (fairly easy access to foundation with no impediments for the accessible walls). So cost is not a great factor (just the boards, maybe $400?). My question is whether it is worth the effort at all with relatively little coverage (and unfortunately the game room that is not currently insulated is inaccessible on the outside)? Would it be pointless to do so if it is not done all the way from the footing of the basement up to the bottom of the siding? Finally, would it be ideal to allow the concrete block to dry out for 2-4 weeks before placing the foam board?
Thanks!
AACO,
First, can you tell us your name? (I'm Martin.)
Q. "Is it is worth the effort at all with relatively little coverage (and unfortunately the game room that is not currently insulated is inaccessible on the outside)?"
A. Only you can answer that question. Insulating a portion of your walls will reduce the heat flow through your basement walls, lower your energy bills, and improve the comfort in your basement somewhat -- but not as well as if you insulated 100% of the area of your basement walls. I'd say it's worth doing, but you may be disappointed in the results. Ripping out the finishes from your basement walls (especially the rooms that feel cold) so that you can properly install interior insulation is another possibility.
Q. "Would it be pointless to do so if it is not done all the way from the footing of the basement up to the bottom of the siding?"
A. No, it wouldn't be pointless. If would just be less effective than doing the job correctly.
Q. "Would it be ideal to allow the concrete block to dry out for 2-4 weeks before placing the foam board?"
A. There is no need to let the concrete block dry out before insulating the blocks on the exterior.
If the fiberglass insulated interior wall has foil facers or there is a polyethylene vapor barrier there is a small risk of moisture wicking up the CMU from the footing to become trapped in the studwall, or even all the way to the foundation sill.
That isn't going to be much of a risk in a high desert location unless the water table rises to the level of the footing, but in regions with higher rainfall it's good to verify the interior wall's stackup is free of true vapor barriers, and that there is an adequate capillary break between the CMU and the foundation sill.
Martin,
Thanks so much for the response. I did read your recommendation in the article, and it is in line with what I have seen from other experts (most notably the Building Science Corporation). My concern is that this advice seems to run contrary to the advice masons give for maintaining this type of foundation. As I understand it, water causes the limestone mortar to break down. If the water is allowed to move through the wall to the interior, then over time the mortar will slowly erode from the interior outward. The foundation is maintained by periodically repointing the interior face with new mortar.
You recommend creating a water-tight seal on the interior face, and the Building Science Corporation recommends creating a capillary break that allows the water to flow through the wall and into drain tile. It is not clear to me how either method would change what happens when lime mortar is exposed to water. A watertight seal on the interior would mean the water is trapped inside the foundation, and prevailing knowledge seems to indicate that would accelerate and expand the breakdown. Utilizing a capillary break would prevent the water from being trapped, but the mortar would still slough off over time and it seems like that would eventually clog the drain tile. Both methods also render the foundation inaccessible, meaning you would have to tear down the insulation in order to do the repointing that is necessary to maintain the foundation.
Even with optimal grading and gutters, some moisture is always going to make its way to the foundation. So my question is really where is the disconnect here? Why do we no longer need to account for the breakdown of lime mortar once we put up a barrier on the interior of the foundation wall? Or is it just that we can't bank on extreme long-term certainty when it comes to home-ownership, so a solution that should last 25+ years is considered effectively permanent?
Chuck,
There's a lot to say about this issue -- more than can fit into a reply to a comment at the end of a long blog.
Briefly:
1. Lime mortars are different from modern Portland-cement-based mortars. Lime mortars have advantages and disadvantages. They are softer and more flexible than Portland cement mortars, but also weaker. Because of this fact, masons are taught to repair walls with lime mortars using a mortar with the same characteristics. That doesn't mean that lime mortar is better than Portland cement mortar.
2. When a lime mortar is applied as a stucco layer, it is often considered "sacrificial," especially if the wall in question has a rising damp problem. Moisture evaporates from the vapor-permeable lime mortar stucco, providing a path for moisture to leave the wall. The salts left behind (the efflorescence) causes the stucco to deteriorate. When the stucco fails, it is re-applied.
3. How to handle interior insulation on a wall like yours depends on judgment, and I can't teach judgment over the internet. When there are signs of liquid water entry, the most robust solution includes an interior French drain, the application of dimple mat on the interior side of the wall, followed by an impermeable layer of some sort and then a layer of closed-cell spray polyurethane foam. If your wall shows no signs of liquid water entry, the dimple mat may be unnecessary. For more information on the dimple mat approach, see "Rubble Foundations."
4. It isn't necessary for water to flow through an old foundation. It's perfectly OK to stop the flow of water, as long as the barriers and drainage details are robust.
5. If your wall is sound and relatively dry, it's OK to install closed-cell spray foam on the interior side of the stone wall. Plenty of spray foam contractors have experience with this approach, and have achieved good results. Whether your wall is sound enough, and dry enough, for this approach is, as I said before, a judgment call.
Clearing up a misconception:
"A watertight seal on the interior would mean the water is trapped inside the foundation, and prevailing knowledge seems to indicate that would accelerate and expand the breakdown."
Keeping the foundation saturated in water doesn't cause it to break down. There are concrete foundations in rivers, (even salt water) that are still functional more than 1000 years after they were installed. Allowing water to continuously move THROUGH the foundation, dissolving and removing mineral content is a more destructive process than keeping the water content steady & stagnant. When the water in the concrete & mortar reaches saturation level of the dissolved materials, and the dissolving process stops, and only restarts if that water is removed and replaced with a fresh charge of water.
A lime mortar parge (where appropriate) is typically good for ~100 years, less if signifcant amounts of water are constantly moving through it, more if the water is NOT moving through it.
My understanding is that saturated with moisture may cause breakdown from freeze spalling - even with no moisture movement through the foundation.
hi. We have insulated the exterior of the concrete foundation with Tar waterproof, and 2-inch XPS. Should we be wrapping a platon / dimple membrane on top of this? Or does the 2 in XPS foam acts as a barrier?
Alpha,
A dimple mat is desirable but not required. For a full discussion of this issue, see "Using a Dimple Mat to Keep a Basement Wall Dry."
Hi Martin, thanks. Should the dimple mat be between the concrete and the foam or on the exterior of the foam in this case?
Alpha,
Once again, I urge you to read the article I linked to. My article discusses the advantages and disadvantages of both approaches.
Hi,
I live in NH and my 100 year old basement is stone.
I recently remortered the joints.
Never had any water issues.
I am having a company give me a quote on spray foam.
Should I paint the basement walls with anything before they spray?
Nitch,
Q. "Should I paint the basement walls with anything before they spray?"
A. No. But you might ask the spray foam contractor (as well as your local building department, if there is one) about code requirements for a coating or thermal barrier on the interior side of the cured spray foam for fire safety.
Hi - Great site; really appreciate the information.
Question: do you have experience / recommendation as to Ghost Shield Lithi-Tek 9500 Concrete Sealer? Planning to seal basement prior to installation of closed cell spray foam.
Are all closed cell spray foams the same product? Or does the product differ by install company.
thanks,
jason
Jason,
Q. "Do you have experience / recommendation as to Ghost Shield Lithi-Tek 9500 Concrete Sealer?"
A. No, I've never used the product. For more information on sealing basement walls on the interior, see these articles:
"Fixing a Wet Basement"
"Wingnut Real-World Testing of Basement Waterproofing"
Q. "Are all closed-cell spray foams the same product? Or does the product differ by installation company?"
A. The quality of the installation job matters more than the brand of foam, so you want to choose a contractor with a good reputation. You probably also care about the environmental friendliness of the foam's blowing agent; for more on this issue, see "Next Generation Spray Foams Trickle into the Market."
The idea you shared for insulating a basement wall is good. You can also use EPS foam sheets, blocks for insulating your property. Great post!
Aaron,
As my article states, "The best way to insulate a basement wall on the interior is with foam insulation that is adhered or attached directly to the concrete. Any of the following insulation materials are acceptable for this purpose: closed-cell spray polyurethane foam, XPS, EPS, or polyisocyanurate."
Our basement has steel 2x3 inch columns every 32 inches on center to reinforce the basement walls which cracked and bowed due to clay soil. How do we insulate in this case? Place the foam boards between the columns and spray foam the gaps between the foam and the columns?
If the soil still moves around you're probably better off using closed cell spray foam on the foundation sufficient for dew point control on R13-R19 batts, and building a non-structural studwall with batts snugged up to your spray foam, surrounding the columns with sculpted & trimmed batts.
How much space is there between the columns & foundation?
The columns are cemented to the foundation walls, so no space.
Bork,
The best insulation job will include both (a) insulation on the interior side of the foundation wall and (b) insulation on the interior side of the steel posts -- with no insulation gaps.
There are two ways to do this: either (1) Build a stud wall, with the interior face of the studs aligned so that the studs cover the insulation you have installed around the steel posts -- so that you end up with drywall that is all in one plane, or (2) Build many short sections of stud walls between the posts, with bump-outs around each post.
In either case, you'll want to use either closed-cell spray foam or cut-and-cobble rigid foam as your initial insulation layer. These types of foam can be supplemented on the interior with additional mineral wool or fiberglass if you want.
Are the posts projecting in to the room from the wall by 3", or is it 2"?
Either way it's unlikely it would need to frame a bump-out around the posts to meet IRC code minimums.
Foam over the posts at the same depth as the wall, and install 3" or more of rock wool insulation over the foam, or at least 1.5" (fire protection) as long as the total R at the post adds up to the IRC code min for your climate zone. (That's R15 continuous insulation in zones 5 & up.)
Assuming a 2" projection into the room, an inch of HFO-blown closed cell foam runs about R7. With 3.5" rock wool batts between the posts you'd have 2.5" from the foam on the post to be flush with the rest of the studwall, which would be anothe R10.7, for R17 total- you'd be good with a trimmed piece of batt.
If it's projecting 3" into the room you'd have only 1.5" of space to fill from the foamed-post to the flush plane. which is about R6.5. That would only add up to R14.5, but at such a low framing fraction , given the R15 + R7= R22 everywhere else it would easily beat code-minimum performance on a U-factor basis. R7 is ample dew point control for R15 fiber in zone 5, but doesn't quite meet the prescriptive in zones 6 or 7. If in zone 5 or 7 bump it to 1.5" (R10-ish) for zone 6, or 2" (R13.5+) for zone 7.
In zones 4 or lower an inch of closed cell between the posts and R13 fiberglass everywhere else (including fiber insulation on the posts themselves) would meet or beat code minimum on a U-factor basis, despite the thin spots in the fiber at the posts.
I am in zone 6 and they project 3" into the room. So it looks like I can put my 2" foam board between and around the posts (R-10). Then build my 2x4 stud wall in front of the 2" foam. That gives me only 2.5" over the posts, but good everywhere else, if I use r-11 fiber between the studs (or another inch of R-5 foam?) . There will also be a 4x4 egress window on this wall, so I will need to frame around that as well. Thanks for the help Martin and Dana.
I like Dana's solution - an inch or two of closed cell spray foam on the walls and into the sides of the columns. Then a stud wall stuffed with fiberglass. Either cut or compress the fiberglass batts over the columns. There is some risk of condensation on the faces of the columns, but I would think this risk is pretty limited, except in very cold climates.
You could also fasten lightweight 3" steel studs between the columns, creating a stud wall with 16" centers. Then fasten rigid foam to the face of all the studs, followed by drywall. Leave the space between the studs open, but sealed at the top and bottom. The space between the studs will get pretty damp and nasty, but that shouldn't hurt anything. Paint the columns first so they don't rust.
The R5 /inch foam is XPS, only warranteed to be R4.5/inch, and over decades will drop to R4.2/inch. It's temporary performance advantage over EPS is due to it's climate damaging HFC blowing agents. Foil faced polyiso will outperform it, and is blown with far more benign hydrocarbon blowing agents, (usually a pentane variant.)
Yes, fill in the extra inch to bring it flush with the rest at the posts with foam board rather than fiber insulation, for that extra R0.25-R2. At 3" most polyiso would still be performing over R15, even after derating for temperature on the colder above-grade section. At just 2.5" it would hit R15 for labeled R.
Martin, I actually went out and bought your book....good stuff, Typically, all my reading nowadays is on an iPad but I wanted a hard copy of that.
We're planning an expansion to our house on Martha's Vineyard. A good portion of the basement wall will be exposed above grade, and for various architectural reasons, we want to see the exposed concrete. I'm planning to fur the interior with studs spaced 1' from the concrete and insulate with closed cell foam. There's one area, however, where a stair is going to descend to the basement against the outside wall...I need to keep the opening as as tight to the basement wall as possible, but I'm concerned about getting insulation against the rim board as there will only be a 1" wide slot available to spray the foam. See detail attached.
Can spray foam be sprayed through the slot and all is good? or do I need to find another solution, perhaps installing rigid foam against the rim board prior to installing the subfloor, or perhaps positioning the rim board 1-1/2" in from the foundation edge and installing exterior insulation against the rim board and subfloor.?
On another subject, without going full bore with Advanced Framing, what do you think about going to 24" O.C. roof and wall framing to reduce thermal bridging effects? Interior finish would be 5/8" drywall and sheathes 5/8" plywood whether or not 16" or 24"...roof deck would probably go from 5/8" to 3/4"
Hugh,
First of all, if you insulate your concrete wall with closed-cell spray foam, I don't know how you will achieve your goal to "see the exposed concrete." (Perhaps you want to see the concrete on the exterior of your house rather than on the interior of the house?)
If you haven't yet installed the 2x4 stud wall on the interior side of the foundation, the obvious solution is to insulate the rim joist with closed-cell spray foam before the stud wall and beam are installed.
If it's too late, and you've gone ahead and installed the 2x4 wall with a beam on top, your access is poor. You may need to drill a series of holes in the beam, and spray the closed-cell foam through the access holes.
Martin....nothing is built yer. still in design phase. We want concrete exposed on exterior, not interior.
The problem with the "obvious solution" may be construction sequencing. Typically, that beam would go in wall-to-wall under the subfloor and the stud wall bottom plate during framing...it might be possible to install just that strip of sub-floor and hope to get someone in just to foam the rim joist if they were doing a job nearby (or perhaps install rigid and use canned foam around it), then slide beam under subfloor. Realistically, I think we'll have to drill holes in the beam and/or the subfloor. How big would holes likely have to be?
Or...if we didn't install the interior furring until the rim joist was foamed, perhaps the larger resulting gap would be sufficient to foam rim joist. See attached revised sketch. What do you think?
Hugh,
Use rigid foam before the stud wall goes up.
see the edit, I just made
Hugh,
Q. "Perhaps the larger resulting gap would be sufficient to foam rim joist. See attached revised sketch."
A. It's hard to say -- depends to some extent on the nozzle you are using and the brand of Froth-Pack kit you intend to buy. I would want better access if I were doing it. A typical Froth-Pack nozzle is shown below.
thanks....I won't be doing it myself...insulation contractor will be...I'll discuss and see if they can do...if not, we'll play around with rigid insulation before they install the beam
@Martin Holladay : Thank you for such an informative article and for keeping up with commenters. I've learned quite a bit from this page alone as there is a ton of misinformation running rampant on the internet today. After reading all the comments posted here I do need some clarification, if you would be so kind to offer guidance?
Some background first: I live near Salt Lake City Utah and just built a home that has the Tuff-N-Dri exterior system installed (R-5 insulation on walls, with a much thicker waterproofing membrane, along with a custom french drain that runs along the exterior footer). We are now finishing the basement and have installed 1" XPS foam (R-5) on the top of the basement floor followed up with 3/4" OSB plywood that is screwed into the foundation (through the XPS). We also installed 2" XPS foam (R-10) along the entire perimeter wall. For the floor and walls, we made sure to seal between each seam, a thin bead of spray foam and followed up with tuck tape.
However, the plumber that ran the plastic drain pipes ran a number of them along the interior of the basement cement walls - and in some cases, every which way along the wall. These pipes are 1/4" to 1/2" from the wall itself and require me to build a soffit around them. However, because they are so close to the interior basement wall, what do you recommend we do to insulate that area to prevent condensation? Here are my concerns:
1) I cannot slide xps behind the pipe because there's not enough space, however, I thought of putting xps just above and below the pipe (on the wall) and then foam the gap between the pipe and the xps, and then foam between the XPS and wood framing (which sits on the cement wall) but that would require quite a bit of foam. I'm willing to do this if it will work, however.
2) If I were to instead use xps foam around the surrounding soffit to keep a vapor barrier continuous, wouldn't that make the pipe very cold and risk freezing, since now the pipe is on the cold side of the cement wall and insulation?
Last, while I read your post above, there was a comment that confused me a bit. We are installing 2x4 framing in front of the 2" XPS foam and were planning to put roxul mineral wool (R-19) in the 2x4 frame to achieve a warmer (or cooler) basement during the right seasons. However, according to a reply above, you mentioned that putting mineral wool in front of the XPS could cause condensation since the XPS would be cold on the interior side, rather than being warm. Did I understand this correctly?
If so, what would be your suggestion to offer more insulation on top of an existing R-10 XPS? Should I cut out another R-10 XPS panel that is 16" wide to fit inside the 2x4 and seal that up against the existing R-10 XPS that was already installed against the wall? If i do that, then I would have an air gap between the sheetrock and the two R-10 XPS foam boards - would that cause condensation or thermal (R) performance issues? Just looking for clarification and ways we could insulate the basement to ensure it stays very warm in the winter as we really don't want it to be cold at all.
Thank you again for your help and time!
Noah
Noah,
If you want, you can use canned spray foam (or even a two-component spray foam kit) to fill the 1/2-inch gap between the drain pipes and the concrete wall. Then proceed with your plan to box in the plumbing pipes -- and stop worrying. (You're right, by the way, that you don't want to install 2 inches of rigid foam on the interior side of the drain pipes -- because the pipes could conceivable freeze, although that's unlikely in your case.)
In the case of your plan to install mineral wool between the studs, you should be safe in your climate zone (Zone 5B) -- especially since your walls have a total of R-10 of rigid insulation on the exterior side of the mineral wool. But if you're interested in the theory, here it is: In very cold climate zones, thin foam and thick fluffy insulation (like mineral wool batts) can be risky, because the interior surface of the rigid foam isn't warm enough in winter to prevent condensation. (Note that the fluffy insulation makes the problem worse, by lowering the temperature of the first condensing surface.) The problem is avoided by installing rigid foam that is thick enough. For more information on the problem, see this article: "Combining Exterior Rigid Foam With Fluffy Insulation."
Thanks Martin, great info! Just a follow up question regarding the mineral wool filling in the 2x4 stud walls. In certain areas, the folks who poured the concrete walls were almost 2" off, so my walls are way off square. I compensated by framing it square so the lvl flooring wouldn't look crooked against the wall. The only problem is that there's a solid inch between the xps foam and the 2x4 framing in some areas. Would it be ok if I filled that gap between the studs and the XPS, with spray foam, and then filled the 2x4 cavity with mineral wool? That being the case, since the mineral wool is only 3.5" thick but the wall cavity would be 4.5 to 5.5" thick in some areas, would it be best if I put the mineral wool against the R10 XPS (which is against the cement wall) which would leave an inch or two gap of air between the sheetrock and the mineral wool - or should I situate the insulation against the back of the sheet rock and leave a gap of air between the mineral wool and the XPS foam? Thanks again!
Noah,
Q. "Would it be OK if I filled that gap between the studs and the XPS with spray foam, and then filled the 2x4 cavity with mineral wool?"
A. Yes.
Q. "Since the mineral wool is only 3.5 inches thick but the wall cavity would be 4.5 to 5.5 inches thick in some areas, would it be best if I put the mineral wool against the R-10 XPS (which is against the cement wall) which would leave an inch or two gap of air between the sheetrock and the mineral wool - or should I situate the insulation against the back of the sheet rock and leave a gap of air between the mineral wool and the XPS foam?"
A. The best approach would be to buy some 6-inch fiberglass batts and compress them. But if you don't want to do that, the gap belongs on the interior side of the mineral wool (so that the mineral wool is touching the rigid foam). That said, you might find it hard to keep the mineral wool batts where they belong.
Is it recommended to tape exterior basement wall insulation? Can Siga or 3M hold up below grade?
Kyle,
I would tape the rigid foam seams. My guess is that Siga tapes or 3M tapes will last a long time, as long as they are protected from sunlight.
Hi Martin, thank you so much for all of your helpful comments on this post, I've been learning tons. I recently moved into a 15 year old house in south-central Wisconsin which has an unfinished basement with concrete walls below grade. The basement has already been framed out with the studs about 1/2 inch from the concrete and fiberglass batting between the wooden studs without a vapor barrier. My wife and I had been planning to simply put up drywall, but reading your posts has lead me to believe that this would be a very bad idea. I'd like to make sure I understand the options based on your feedback to others.
Option 1: Remove the studs, put in rigid foam and tape, put up new stud wall. I'm concerned about the complexity of this, since electricity and plumbing has already been run through the studs.
Option 2: Spray foam. Would this even work with the studs as close to the concrete wall as they are?
Option 3: Remove fiberglass batting, and fill the stud voids with rigid foam, potentially filling up gaps behind the studs with canned spray foam. This would be the "cut-and-cobble" method, correct?
Option 4: Put up drywall without a vapor barrier. Would this allow sufficient air flow to prevent mold growth?
We've only been in this house for 2 months, so we aren't sure how much condensation if any we get in other seasons. So far it has seemed dry, though we do have a dehumidifier running.
Again, thank you so much for all of your help and knowledge.
Colin,
Option 2 is by far the best. In Option 4, your logic is backwards. You wrote, "Would this allow sufficient air flow to prevent mold growth?" In fact, air flow is the enemy. The more air flow, the greater the chance of mold. That's why you want spray foam -- to prevent the moist indoor air from contacting the cold foundation.
Thanks for the quick response. I figured that would be the case, was trying to see if a less expensive option was feasible, but looks like I should start getting some quotes for the work.
I'm deeply puzzled as to whether I should try to seal the concrete of my basement wall (on the inside) before attaching interior foam insulation? Looks like a previous owner tried to do that and it just bubbled the paint away and now is dropping sandy concrete along with the peeled paint on to the floor. If it should be sealed, with what? And is it possible that the surface of my already moisture degraded wall is just too uneven to seal? The wall itself is in very good condition, but it sure looks like the previous paint job locked the moisture behind it and caused the concrete to soften and crumble? Thanks for the help!
Steven,
It's hard to assess what's going on in your basement without a site visit. Briefly:
1. It's hard or impossible to waterproof a below-grade concrete wall -- and in most cases any attempt to do that happens on the exterior, not the interior. For more information, see these three articles:
"Installing Basement Waterproofing from the ‘Negative’ Side"
"Wingnut Real-World Testing of Basement Waterproofing"
"More Wingnut Testing of Basement Waterproofing"
2. You don't want to insulate the interior side of a basement wall if there are signs of liquid water entry. If you have liquid water entry, you have to solve that problem before insulating. If you have no water-entry problems, however, you can proceed with your plan to install interior rigid foam insulation.
Thanks for your quick response. I will include a couple pictures to show and tell. This basement was built in 1925 and they poured concrete about 4 feet up and then went the rest of the way up with large concrete blocks. The previous owner tried to paint the walls and then basically covered the lower 4 feet of poured concrete with polystyrene and then covered just that with sheetrock. It seams to me that the paint held pretty well on the upper block portion of the wall, but the old school concrete on the lower half absolutely did not adhere and perhaps just trapped moisture and caused it to disintegrate on the surface. The lower poured 4 feet would be VERY difficult to paint / seal with the amount of rough surface.
The great news is, that this basement has proven itself to be very dry in the last two years that we have owned it (and we've been through some REALLY wet times in those two years). So I feel comfortable taking your advice in (2) and proceed with installing rigid foam.
I had gotten pretty confused regarding trying to seal the walls by this fine homebuilding video https://www.finehomebuilding.com/2016/07/07/insulate-basement-part-1
So I greatly appreciate you confirming what I suspected about the futile effort to seal from the inside.
Steven,
In the video you linked to, Justin Fink shows a few tricks to limit moisture entry or water entry into a basement. He said, "“While I don’t recommend that [waterproofing paint] as your first line of defense, it can be a helpful strategy in combination with other systems.” That's the key point.
In my mind, the use of waterproofing paint (like Drylok) is a last-ditch effort when other efforts have failed -- and waterproofing paint has limited effectiveness. In an older house like yours, such products occasionally have uses -- but not on crumbly sandy concrete.
If you don't have structural concerns -- that is, as long as there is no chance that your foundation is actually failing -- and as long as there is no water entry, go ahead and insulate your wall with rigid foam.
Thanks Martin! Sounds good, I will proceed. Carpe Diem!
I understand the logic of these methods and am totally on board with NO fiberglass batt in a basement. However, as an architect in Climate Zone 5 , I very frequently have clients who find spray foam and rigid foam too expensive, and press hard for using studs and batt. Someone suggested draping Tyvek down the inside of the concrete foundation wall, holding the 2x4 studs 1" away from foundation wall, filling with R-batt, and installing GWB over it on the interior. Does this make sense to anyone? Can anyone run a WUFI calc on this type of wall?
Laura, no need to run a WUFI--that is a terrible assembly for a foundation wall. Tyvek is vapor-open, so moisture will move right through it in either direction. Fiberglass is air- and vapor-permeable so warm, moist interior air will diffuse or leak into the wall assembly, move right through and accumulate at the concrete.
If clients find foam too expensive when finishing their basements, they should not finish their basements. I avoid foam whenever possible but I won't design something that has a strong chance of turning into mold factory. A single layer of polyiso behind stud walls insulated with fiberglass turns it into a reasonably safe assembly, with minimal added cost.
Laura,
You wrote that you are "totally on board with NO fiberglass batt in a basement." Then you suggested using a product called "R-batt." I'm not familiar with the product. What is "R-batt"?
Pardon my typo. I meant to write R-13 fiberglass batt.
The frequency at which our clients (many low-income housing providers or DIY-ers) reject our design of R-15 rigid XPS or spray foam and press for the more familiar/cheaper studs and batts, is what prompts me to ask.
As Michael Maines suggests above, perhaps the polyiso or XPS behind the studs and batts would perform okay? If so, what thickness of each do you recommend?
Thanks for your swift reply.
For climate zone 5, you should aim for at least R-15 of continuous insulation, or R-19 in the studs. If you aim for all of the R-value in the foam, you can skip installation of batts in the walls and that provides some simplification of construction and cost savings.
Three inches of XPS gets you there, but XPS has a relatively high GWP and 3" of XPS is not vapor permeable enough to allow any drying.
Four inches of EPS also gets you there at a lower GWP. Recycled EPS is often available in/near urban areas and can be very inexpensive.
2.5" of polyiso also get you there, and you can also often find recycled polyiso. Fleece faced is better than foil faced for basement walls.
One common assembly is to hold the studs 1"-1.5" away from the foundation walls and spray the space behind and between the studs with closed cell SPF. Then, fill the studs with R13 batts to bring the overall total close to R19 and the overall U-value within code. This is a relatively simple construction technique that doesn't require any significant changes in typical trade practices.
I agree with Michael's comment above - if you can't afford to do it right, don't do it. Why would you want to build a basement knowing that it will inevitably grow mold and require redoing within 10 years or so? That is a tremendous waste of resources. Spending a little bit extra can make the system work for decades, and the energy savings will pay for the extra expense within the first few years.
I agree that 3" of XPS or the equivalent negates the need for studs and batts, but that is the very design my clients reject due to cost. Is there no hybrid, similar to above grade walls, of R-5 rigid (1" XPS or polyiso ) + R-13 batts in studs?
Laura,
Martin addresses that in the above article. You can do it but it isn't risk-free.
>" agree that 3" of XPS or the equivalent negates the need for studs and batts, but that is the very design my clients reject due to cost. Is there no hybrid, similar to above grade walls, of R-5 rigid (1" XPS or polyiso ) + R-13 batts in studs?"
XPS has no place in any "green building" discussion. It's labeled R5/inch but only warranteed to R4.5/inch and will in fact eventually drop to R4.2/inch (if the building survives long enough) as it's climate damaging HFC blowing agents slowly diffuse out. It's far and away the least green insulation material in common use today:
https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png
Using 1.5" of EPS or 1" of foil faced polyiso (about half the environmental impact of closed cell spray foam) is still a hit, but no a horrific hit. That provides sufficient dew point control for R13 on the above-grade portion of the wall, and is vapor retardent enough to keep ground moisture from destroying the studwall as long as the interior finishes are somewhat vapor open, such as standard interior latex paint on wallboard (and NOT vinyl or foil wallpaper.)
Using an inch of EPS (not polyiso) under the bottom plate of the studwall as a thermal and capillary break keeps ground moisture from getting at the bottom plate. If using polyiso on the walls, keep the bottom edge a half inch off the slab, since polyiso can wick ground moisture directly from the slab even if it's not visibly damp.
I using batts, unfaced is safest. In the event of a minor flood kraft facers can wick the moisture higher. Foil facers are true vapor barriers and could trap ground moisture in the studwall, even with rigid wall foam limiting ground moisture movement.
“[Deleted]”
Hello all, looking for some input on insulating my basement. Home is new construction in New Jersey, and builder installed a drain channel? around the perimeter of the basement - 1/2 thick dimples off the wall and goes up the wall 2-3 inches. What are my best options for insulating? Township requires R15 total insulated value.
1. Install dimple membrane on entire wall surface, then 2" Foamboard over membrane, then and regular batt insulation between stud wall
2. Frame the basement 6+ inches off the exterior walls and use regular batt insulation
3. Foambard flush to wall and stop at top of drain channel
Ruling out spray foam as the cost will likely be out of budget to do 1,200 sq ft.
Your guidance is much appreciated!
Chris
Chris,
The answer depends on whether there are any signs of water entry. If your basement is dry, with no signs of water entry, Option #3 is fine. (You can even install a custom strip of rigid foam to cover the dimple mat, using a thinner type of foam than the foam used for most of the wall. If you do this, tape the seams to keep everything airtight.)
If there are any signs of water entry, go with Option #1.
Option #2 is a bad idea and would never make any sense.
For more information, see "Using a Dimple Mat to Keep a Basement Wall Dry."
Martin, I can't thank you enough for your reply, and thank you for the wealth of information and support you provide here!
We have been here two years and no water anywhere in the basement, so I am thinking Option #3 is the winner.
Ok, I think I understand what you mean with the layers. Should I do 1/2 foam first so its a clean match to the dimple mat, and then 1.5" directly on top of that, or 2" first then 1.5" in front of dimple? Maybe I am overthinking it, but just want to make sure I am applying to get the most benefit.
Chris,
Q. "Should I do 1/2" foam first so its a clean match to the dimple mat, and then 1.5" directly on top of that, or 2" first then 1.5" in front of dimple?"
A. Either way will work.
Q. "Maybe I am overthinking it?"
A. Perhaps.
“[Deleted]”
Thanks for the great information. In looking at the wall detail image, I have a few questions.
1) In looking at the diagram of the wall detail, I see that a sill sealer under the bottom plate is recommended. In most cases, I see the PT board attached directly to the concrete, with a bead of spray foam between where the rigid and PT meet. What type of sealer would you recommend? From the diagram, it seems as thought the sealer should be completely under the PT board.
2) I have about 1.5" or so of horizontal concrete at the top of the wall. Should I just spray foam that section? Then my fire blocking would go on top of that section, extended so that it is above the top plate?
3) Finally, my stair stringer is about 1/2" from the wall. Can I add my 2" rigid foam above and below the stringers and just spray foam the half inch space between the 2 sections of rigid foam. There will be a wall around the bottom of the stairs with a little closet underneath.
Thanks in advance.
>"1) In looking at the diagram of the wall detail, I see that a sill sealer under the bottom plate is recommended. In most cases, I see the PT board attached directly to the concrete, with a bead of spray foam between where the rigid and PT meet. What type of sealer would you recommend? From the diagram, it seems as thought the sealer should be completely under the PT board."
In my area people are installing sill gaskets (designed for the tops of foundations) under the bottom plates as a capillary break for any walls going on an uninsulated slab. I personally prefer putting an inch of EPS foam board under NON-pressure treated bottom plates of non-structural walls, since that keeps the wood warm enough in summer to avoid mold issues even when ventilating the basement with humid summertime air.
>"2) I have about 1.5" or so of horizontal concrete at the top of the wall. Should I just spray foam that section? Then my fire blocking would go on top of that section, extended so that it is above the top plate?"
Cutting 1.5" thick foam board strips (the same thickness as 2x foundation sill lumber) and caulking them in place with polyurethane caulk or foam board construction adhesive works fine. But if you're otherwise planning on spray-foaming the band joist and top of the foundation sill (rather than cut'n'cobbled foam board), sure go ahead and spray foam over the top of the foundation & over the top of your wall foam too.
>"3) Finally, my stair stringer is about 1/2" from the wall. Can I add my 2" rigid foam above and below the stringers and just spray foam the half inch space between the 2 sections of rigid foam. There will be a wall around the bottom of the stairs with a little closet underneath."
In most cases, yes that works just fine, as long as there isn't visible efflorescence on the foundation wall in that area indicating high moisture levels in the concrete or CMU. If the masonry is damp, install thin sheet foam between the stringer & masonry. Even 1/4" fan-fold XPS sold as siding underlayment is better than sealing a non-pressure treated stringer to a damp wall.
Thanks for the information Dana...really appreciate the quick reply. I thought of a few other questions.
1) I plan on having a mechanical room for my furnace, hot water heater, etc. Would I insulate those walls in the same way as the rest of the basement? Is it better to the concrete foundation bare there and somehow insulate just the interior mechanical room walls? I was actually going to leave my stud bays empty in the rest of the basement, so I'm not sure what the choice would be for insulation for interior walls.
2) What about insulation between the basement ceilings joists, a good idea or not neccessary? If helpful for comfort and sound to the first floor, would roxul be a better choice that batt insulation?
Thanks so much for you advice...greatly appreciated!
1) I plan on having a mechanical room for my furnace, hot water heater, etc. Would I insulate those walls in the same way as the rest of the basement? Is it better to the concrete foundation bare there and somehow insulate just the interior mechanical room walls? I was actually going to leave my stud bays empty in the rest of the basement, so I'm not sure what the choice would be for insulation for interior walls.
2) What about insulation between the basement ceilings joists, a good idea or not neccessary? If helpful for comfort and sound to the first floor, would roxul be a better choice that batt insulation?
Thanks so much for you advice...greatly appreciated!
Toley,
You didn't tell us your geographical location or climate zone -- it would be useful to know where this house is. In general, a mechanical room should be inside of a home's thermal envelope -- so you need to insulate the basement walls (the exterior walls) of your mechanical room.
Insulation between the floor joists above the mechanical room is optional, but such insulation can be part of a noise-reduction strategy (if that's what you're aiming for). Other elements of the assembly -- including resilient channel or multiple layers of drywall -- can also be helpful in noise reduction, so you have to consider the assembly as a whole before deciding on the details.
Note as well that you need to provide combustion air to the mechanical room if the room contains any fuel-burning appliances that aren't rated for sealed combustion -- for example, an atmospherically vented gas-fired water heater or furnace.
Thanks Martin. I'm in climate zone 6a. I will plan on rigid foam on the exterior walls of the mechanical room, as with the rest of the basement. There is a room in the basement that was built before I was the owner of the house. I believe there is poly against the concrete, then a stud wall with batt insulation, and poly over the insulation. I haven't torn the wall apart yet, but it would probably be best to tear that out and do the rigid detail correct? Also, I do notice near the walkout section of the basement, there is frost on the concrete in the winter. Would the 2 inch rigid foam on the interior take care of this, or do I need to address this from the exterior? Thanks in advance.
Toley,
Q. "I haven't torn the wall apart yet, but it would probably be best to tear that out and do the rigid detail correctly?"
A. Doing things correctly is always the best approach -- but when it comes to deciding whether you can afford to tear everything out and do it again, only you can figure that out.
Q. "I do notice near the walkout section of the basement, there is frost on the concrete in the winter."
A. I'm going to assume you mean that the frost is on the interior side of the concrete wall.
Q. "Would the 2 inch rigid foam on the interior take care of this?"
A. It would probably eliminate the interior frost, but it wouldn't be enough to meet minimum code requirements (R-15 for basement walls in your climate zone).
Q. "Or do I need to address this from the exterior?"
A. As the article on this page explains, you can install the basement wall insulation either on the interior side of the wall or the exterior side -- your choice. Insulation in either location will work to eliminate the frost problem and any potential condensation problem, as long as the insulation is thick enough, and is installed according to the recommendations in this article.
Thank you!
The fact that people are commenting on this article 8.5 years past its publication is testament to the fact that it is beyond helpful. This single post and corresponding comments are worth the cost I paid for an annual GBA membership. So, Merry Christmas to me. I've spent the entire day in rabbit holes - my day job can wait, right?
Background: I am in the early stages for a basement finish and want it to be as comfortable as possible - plans are drawn but I want to get out in front of the builder on some of the things I know they'll take shortcuts on if I don't do it myself, like sticking batt insulation in the wall cavities. Our house is a 1956 ranch in NE Kansas (zone 4) where our soil is more akin to clay and is very expansive and the houses in this area were built very low with terrible grading to create positive slope. Suffice it to say we and the previous owners have invested tens of thousands of dollars into stabilizing and getting this basement dry - I believe we have achieved that goal so now it's all systems go. Worth noting is there are no efficiency standards adopted by Kansas, so insulating the walls is not code, but as I mentioned I want to make it as comfortable as possible and follow best practice in the meantime.
I plan to use EPS (sadly 1.5" seems to be all I can find nearby) on the walls and in the rim joists and seal off properly with expanding foam, caulk, and tape.
1. We have an interior drain tile system that leads to two sumps - would it be best to put a dimple mat on the wall to be sure if any water does penetrate, it finds its way to the sump wells? If I do this is there a way to seal off the bottom of it, to make sure the water drains into the crack between the wall and the floor? Would flashing tape between the floor and rolled up the dimple mat work for this?
2. We have perimeter bracing that consists of w-steel columns (4" flanges) poured into the concrete floor and stabilized using the overhead (main floor) system around the entire basement spaced mainly at 4' intervals. This presents multiple issues to finishing the space, not the least of which is having to pull framing out from the perimeter walls between 4" (best case) and 7.5" (worst case) depending on how it lays out. I would like to get the backside of the finished space sheetrock as close as possible to the face of these columns - but I don't want them to be contacting it directly - so what if I wrapped them in 1/2" or even 1/4" rigid foam? Would that be enough of a thermal break?
3. Based on #2, it isn't hard to imagine some of our walls have bulged and are nowhere close to flat and plumb, making it hard to affix a rigid foam panel without having various gaps behind it - is this a significant issue? (FWIW we've had a structural engineer inspect the foundation, so overall integrity is not at issue)
4. Would it be better to hold rigid foam panels off the floor with a gap large enough that I could fill this entire lower cavity with expanding foam once the bottom wall plate is fastened?
5. If we finish the ceilings and walls with sheetrock, does that eliminate the need to cover any spray or rigid foam applied in the rim joist bays?
6. I could easily use 4" rigid foam insulation if I wanted to based on the wall bracing, but cost and availability seems to be an issue at that width. I'd like to use at least 2" minimum (2.5" preferably) if I can find it. Big box stores only seem to carry up to 1.5" EPS but can find 2" XPS more or less anywhere. I've looked online briefly but haven't been able to locate any reclaimed stuff nearby. Is there a reliable website that lists where this might be found?
Thank you for these articles and all the expertise - very helpful to the weekend warriors out there like myself.
Michael
Michael,
You may get more answers to your questions if you post them as a separate topic in the Q&A section, link at the top.
Regarding question 1, there are a bunch of 'trim' options designed to direct water from a dimple board install down below the floor slab. Some need to be poured into the slab, and I think they probably work best, and there are some that basically act as a faux baseboard that you seal so the water stays 'behind' the moisture resistant baseboard. Ask your dimple board provider as there are a lot of different options. If you google 'hydro channel' or 'dry-up system' you'll see two examples of different approaches.
Sam - thanks for the reply and info. I will take my inquiry over to the Q&A forum as well. Cheers!
Two questions based on the article.
Statement 1 - “In fact, you don’t want to encourage any moisture to enter your home. Your concrete wall can stay damp for a century; that dampness won’t hurt the concrete.”
Question 1 - I have seen a few videos on YouTube of basement CMU walls being “mushy” and crumbling by being tapped with a hammer. Was this not caused by the water? What else caused this this structural decay?
Statement 2 - “No. Basement wall systems should never include any polyethylene.“
Question 2 - I need to insulate my interior CMU basement walls to R10 and would like to use 2” of XPS. With a perm rating of approximately .55, am I getting too close to a vapor barrier and the problems that it would cause? If so, I could use EPS which has a higher perm rating, but I would need to increase to 3”. Thoughts?
And I agree with Mike. The answers that some people post are read by people like me almost a decade later. Its like a Wikipedia of building knowledge. It's also reassuring when you see that an older article has been "updated on ..."
Jacob,
Q. "I have seen a few videos on YouTube of basement CMU walls being mushy and crumbling by being tapped with a hammer. Was this not caused by the water? What else caused this this structural decay?"
A. I'm not sure, but CMUs (especially older ones) vary widely in quality. In the old days, they weren't necessarily made of what we call "concrete" -- that's why they were called "cinder blocks." In some cases, they were made from cinders.
Q. "With a perm rating [for XPS] of approximately .55, am I getting too close to a vapor barrier and the problems that it would cause? If so, I could use EPS which has a higher perm rating?"
A. A low perm rating for the rigid foam layer won't cause problems. Remember what I wrote -- that statement you quoted? "You don’t want to encourage any moisture to enter your home." So a high permeance is OK. But EPS is a better choice than XPS for a reason unrelated to its permeance -- XPS is manufactured with a blowing agent that has a high global warming potential. EPS is more environmentally friendly than XPS.
Another advantage of EPS: higher R-value per dollar spent.
A good overview of what causes concrete to deteriorate: https://www.cement.org/docs/default-source/fc_concrete_technology/durability/is536-types-and-causes-of-concrete-deterioration.pdf?sfvrsn=4
>" I have seen a few videos on YouTube of basement CMU walls being “mushy” and crumbling by being tapped with a hammer. Was this not caused by the water? What else caused this this structural decay?"
Water will dissolve some amount of mineral from the CMU & mortar, but that process stops when the water is saturated. If water is allowed to pass through the CMU and evaporate toward the interior, the water replacing it dissolves more mineral. When there is efflorescence on a CMU wall it means water is dissolving something out, then re-depositing it on the surface as the water evaporates. Slowing/blocking that evaporative process with a vapor barrier or lower-permeance foam board slows/stops the process.
Cementicious materials like concrete can last a very long time even fully immersed in water, as long as the water isn't constantly moving through the material displacing the dissolved-mineral laden water within the material with unsaturated water. (They build bridge & pier footings & foundations with concrete, after all.)
Traditional "cinder block" was made using coal cinders as aggregate instead of gravel, or at least for some portion of the aggregate. I've been told that they acidic coal ash is incompatible with the basic lime/cement binder and that the two essentially corrode each other over time and in the presence of moisture. Whether true or not, most of the old CMU foundations that I have encountered with severe corrosion/rot issues were made from this older, true "cinder" block. The only other CMU that has been problematic locally are the blocks made on-site using local sand. Making block on-site was popular with Sears kit houses and some others. Local beach sand is very salty and salt is a well-known corrosive agent for concrete. Both of these CMU variants suffer far more in freeze/thaw cycling than in more constant conditions, even constant wet conditions. I guess this is another argument in favor of exterior foundation insulation and waterproofing, when possible.
Hi! I tore down our fiberglass plus vapor barrier wall after seeing lots of moisture trapped behind it. That led me here. Going to do 2" rigid foam. First I broke off all the iron spikes and plan to fill those holes with hydraulic cement. Then I'm unsure about painting a watertight seal over the concrete first? Or just foam board directly onto the base concrete and skip the watertight paint? Thank you so much!
Ande,
If you know that your basement doesn't have any water entry issues, you can go ahead and insulate by installing the rigid foam directly against the concrete.
If your basement has water entry issues, you can't insulate yet. For more information, see this article: "Fixing a Wet Basemet.". If you're thinking of installing a product like Thoroseal, UGL Drylok, or Xypex, you should read that article first.
Martin! Thank you so much for the quick response! I can't thank you enough. I think I'm good on the inside. I just have a crappy window seal which I'll fix with a ton of caulking soon when spring begins. I've been letting the wall dry out and I think it's okay. EXCEPT for this sudden new problem I exposed just moments ago. Someone decided to have no insulation in my rim joist next to the furnace combustion and vent. It appears to have been condensing and dripping onto my sill plate for possibly years. I'm scared and just started drinking already. Is there a way to cut out a two foot chunk and replace it without cutting through to the outside? I could rent a jack post and hold the floor up for a day? Or the joists are running parallel to this plate in question so maybe it's okay? Please and thanks for any ideas!!!!!!
Ande,
First of all, you shouldn't perform any home repairs if you've been drinking. Wait until tomorrow.
If the sill plate sections are punky -- you can test them with an icepick or an awl to find out -- the damaged lumber will have to be removed and replaced with new pressure-treated wood. If this type of work is new to you, you should hire an experienced carpenter.
Thank you, Martin!
Appreciate your concern. I got a little dramatic there but no worries. Feeling more optimistic and confident in fixing it. I have the nearby joist braced up for peach of mind, going to carefully cut out and replace a good 2-3' chunk, screw and glue it in place, and then cut and replace the leaky furnace vent line. I'll share success pictures soon when I'm done. Thanks!
I am designing a retrofit of an old concrete block pool hall to use as apartements in climate zone 5. We are aiming for a total of R20 for the wall assembly. The walls involved are all above grade, but I think the principles from this basement wall discussion would still be the ones to apply. The budget is tight as this is for affordable housing. The owner/builder was planning to simply put studs inside the block and add fiberglass batts, with new vinyl siding on the exterior. Since this does not involve the challenges usually involved in insulating the exterior of an existing basement wall, I am thinking R5 polyiso over the exterior of the block, with R15 batts inside the stud cavity inside. I know you wouldn't like the batts at all in a basement wall, but this is all above grade. I realize that we could do all of the insulation on the exterior, but I expect the foam is more expensive than batts and he would need to get longer fasteners to secure furring strips for mounting the siding to the foam. Is this a good way to go? Would construction be simpler and performance nearly as good if we did the foam on the inside, with studs and batts inside that?
Hallie,
Your plan to install R-5 exterior rigid foam, along with interior R-15 batts between a stud wall, sounds fine. You should be aware of the controversies surrounding the installation of vinyl siding over furring strips, however. For more information, see "Can Vinyl Siding be Applied Over Furring Strips?"
Needless to say, you need to come up with a water-management plan. You'll need to figure out a way to flash the windows, and you'll need to integrate your window flashing with your water-resistive barrier (WRB).
Installing continuous rigid foam on the interior side of the concrete block wall would be an inferior approach, but it would work.
Martin,
How inferior is the interior foam? Several neighbors of mine have injected SPF (presumably closed cell) between their plaster interior walls and (structural) brick exterior walls. This doesn't seem significantly different from interior board foam assuming the spray foam is closed cell. Is spray foam between plaster and brick risky?
AO
Thanks Martin! I haven't worked with this contractor before- the window flashing may be the deciding factor for him. The discussion about vinyl siding over furring strips was interesting too. Thank you for the expertise as always!
Hi! Thank you for your help in advance. I am seeking an opinion regarding a wall assembly (New York Zone 5). The area is a basement (rather a walkout, please see a picture) build out of cinder blocks. Cinder blocks are covered with tar paper which is a bit deteriorated (simply missing) in various spots. Peeling it off would be labor intensive and cost prohibitive. Looking at 70 years old walls, I can also tell that water has entered the area in the past through the walls. I am looking to build a french drain inside and staple a dimple mat directly on the tar paper so the water can escape to the drain. Next I am considering foam boards (perhaps FOAMULAR NGX which can supposedly get wet, according to the manufacturer), planning to spray foam sill plate\rim joist areas. Then I would like to build studs and likely rockwool in between them. Will top it with sheetrock and paint. Questions:
• Any red flags regarding the aforementioned wall assembly? Am I missing anything?
• Can the foam boards get in contact with the concrete slab at the bottom or should I have some other material between them?
• Floor joists rest on cinder blocks, hoping that air space crated by the dimple mat will not lead to mold production and then floor joists rotting, any concerns here? Second story is a brick as you can see (there is no flashing etc.)
• If done well would foam board “stop” or significantly limit moisture (and musty smell) transmission from cinder block walls to the area interior? The foam manufacturer claims Air Permeability at 75 Pa (L/s.m2), Negligible (0.001)
• On the outside, where exposed, can I cover this wall with faux stone veneer, which supposedly is not water preamble, hence I would run a risk of trapping moisture either in the cinder blocks or where the dimple mat is. Is this a problem or can cinder block be almost permanently wet? Trying to achieve an effect presented on the second picture.
Thank you!
Sebastian,
You have a great number of questions--so many that your questions are best posted in GBA's question-and-answer section. Here is the link: Question-and-answer section. If you post your questions there, you'll get answers from a variety of experts.
I'll start off with one comment: Your basement has concrete block walls with signs of water entry. This type of water-entry problem is best addressed by exterior solutions, not interior solutions--so installing an interior French drain and dimple mat probably shouldn't be the first step. (Those solutions may or may not work, depending on the amount of water entry your basement is facing.)
For more on this issue, see "Fixing a Wet Basement."
Thank you for your reply, I will post in the recommended section and simplify. And yes, the exterior solutions are definitely the best, but they are cost prohibitive. Trying to apply the best solution possible given the circumstances. Not expecting miracles. At the end, I still plan to install a central dehumidifier, but perhaps I can minimize the "wet" problem.
@Martin Holladay,
Hi Martin. Thank you for the helpful information. I am in the process of finishing my basement (still) and just tore down an existing room that had batt insulation against the concrete wall, followed by poly and paneling. This setup was 40+ years old and the batts actually looked great...though smelled very musty. I will be adding 2" rigid foam and will either leave the stud bays empty or will use Roxul...haven't decided. In that room, I am debating on what to do with the ceiling. It looks like it was insulated with batt insulation, with a layer of poly, then paneling. Should this also be removed? I might have asked this question previously, but what is your recommendation for insulating the ceiling...needed or no? Thanks in advance!
Toley,
There's no need to tear down the ceiling unless you want to, especially if your wall insulation seems dry. The existing fiberglass insulation can stay.
But in general, basement walls need to be insulated, but basement ceilings don't need to be insulated. Ideally, your basement is part of your indoor conditioned space. Since you have conditioned space on both sides of your basement ceiling, that ceiling doesn't need to be insulated.
@Martin Holladay,
Thanks for the information Martin. I ended up having to take the paneling off the ceiling because I couldn't get at the rim joist, which isn't insulated. Most of the batts stayed in place between the ceiling joists though, so I guess I will leave them. Do you have any suggestion on whether I should seal the cove joint between the poured wall and floor? I have seen conflicting information on this. Thanks again!
Toley,
What product are you thinking of using? Caulk? Something else?
If you have a big crack there, caulk makes sense as an air sealing solution. (Exterior air can enter a house through foundation cracks.) But caulk won't stop water entry because the hydrostatic pressure leading to water entry overwhelms most common caulks.
@MartinHolloday,
Hi Martin. I was thinking of just using Sika polyurethane caulk. There has never been any indication of any water infiltration from this gap. It is very small and is around the entire perimeter where the wall meets the floor. I hope to be adding my rigid foam soon and just wasn’t sure whether I should seal that little gap first. Thank you for your time!
I have insulated my basement with 2” rigid and plan to build 2x4 walls. However, my basement stairs on one side are built against the poured foundation. There is about 1/4” between the stringer and the wall. I added 2” rigid under the stairs and spray foamed between the stringer and wall. I don’t have enough room for 2x framing above the stringer. I am hesitate to use furring strips because I don’t want to put holes in the wall. Can I glue drywall or shiplap to glued rigid?
Toley,
Q. "Can I glue drywall or shiplap to glued rigid foam?"
A. Yes. Forty years ago, I glued drywall to glued rigid foam in a crawlspace, and everything is still tight and perfect. The two important points are:
1. Choose an adhesive that is compatible with rigid foam. (The information should be provided on the tube of adhesive.)
2. Apply pressure while the adhesive sets. (I used scraps of plywood and OSB against the drywall, held in place by long 2x4s and 2x6s and were wedged tight against the opposite wall.)
Thank you Martin. I have used Loctite PL 300 foamboard adhesive and can’t budge the rigid I have already put up. I will use this on the drywall as well in the small area above the stairs…thanks for the info!
Hi Martin...do you remember the thickness of drywall you glued directly to the glued rigid? What would you recommend? Thanks so much!
Toley,
I used 5/8-inch drywall (which is fire-rated) because the foam was adjacent to a horizontal forced-air furnace.
Thanks Martin. Mine will be along the basement stairs and under the basement stairs…just something to cover the rigid. Think 1/2” would be ok?
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