Image Credit: Andrew Michler
More Musings of an Energy Nerd
UPDATED on May 22, 2015
Many green builders want to build a foam-free house — that is, a house without any rigid foam insulation or spray foam insulation. The reasons behind this desire vary: some builders dislike foam because it is manufactured from petroleum; some because of off-gassing worries; some because of foam’s relatively high embodied energy; some because of the negative environmental effects of the blowing agents used to make foam; and some because they prefer to use natural building materials like straw bales.
I believe that the use of some types of foam insulation is often defensible, and that there are valid counterarguments to many anti-foam positions. However, I’m not going to debate these issues in this article. Instead, I’m going to provide recommendations for those who want to build a foam-free house.
Avoiding foam is usually easy
To build a foam-free house, designers and builders can choose from a variety of foam-free approaches to accomplish the following tasks:
- To insulate a vented cathedral ceiling;
- To insulate an unvented cathedral ceiling;
- To insulate an attic floor;
- To insulate above-grade walls;
- To insulate a floor above a ventilated crawlspace;
- To insulate the walls of a crawl space or basement;
- To insulate a slab on grade or a basement slab; and
- To seal air leaks.
Some of these tasks — for example, insulating an attic floor — are easy. Others — for example, insulating a slab on grade — are more challenging.
Vented cathedral ceilings
As long as the rafters are deep enough, vented cathedral ceilings can be insulated with a wide variety of fluffy insulation materials, including cellulose, denim batts, mineral wool, or fiberglass.
For more information on ways to detail vented cathedral ceilings, see How to Build an Insulated Cathedral Ceiling.
Unvented cathedral ceilings
Creating a foam-free unvented cathedral ceiling is challenging.…
Weekly Newsletter
Get building science and energy efficiency advice, plus special offers, in your inbox.
This article is only available to GBA Prime Members
Sign up for a free trial and get instant access to this article as well as GBA’s complete library of premium articles and construction details.
Start Free TrialAlready a member? Log in
37 Comments
I think houses on piers are a
I think houses on piers are a terrible idea when & where a foundation of some type directly on the ground is feasible, simply because the base temperature under the floor and insulation is far lower when the house is on piers. (In northern climes) That said, though, my brothers house in Fairbanks and the homes of most (all?) of his friends & neighbors are on piers, floating on the permafrost, and they work.
Response to Bob Irving
Bob,
Thanks for your comments. Two other things to think about for builders considering a house on piers:
1. If the occupants ever develop a handicap, you have to install a very long wheelchair ramp.
2. In a cold climate, it can be hard to keep the pipes from freezing.
rock wool on the xterior of the foundation
While insulating the foundation on the above grade portion of the foundation with rigid rock wool can work, there are issues below grade. Internet scuttlebutt has it that below grade exterior rock wool tends to get thinner over time from both frost-compression and soil pressure, and that it's NOT a subtle issue.
FoamGlas, loose pumice backfill, autoclaved aerated concrete block, AirKrete, etc all have much more compressive strength, making them better suited for the below grade portion. But they all will suffer performance hits from liquid moisture that foam doesn't have, making bulk water management even more critical.
Our unvented, foam-free cathedral solution
I'm not a builder, but the foam-free, unvented cathedral system we used in our house (built last year) was designed by a well-respected Vermont energy consultant and approved by our contractor, who takes a pretty conservative approach when it comes to trying unproven ideas. So at the risk of learning something that might keep me up at night, I'll share it here: From the outside: Standing seam roof, Ice and Water Shield, Advantech, 18" of dense pack cellulose, Siga "smart" vapor barrier, sheetrock. The rafters are 2 x 10's and then we hung 2 x 6's 2 in. below those. The house is in Central Vermont. Could this be added to the list of acceptable foam-free unvented cathedral ceiling methods?
Response to Randy George
Randy,
There are two problems with your roof assembly:
1. Many building scientists point out that the use of air-permeable insulation (like fiberglass or cellulose) in unvented cathedral ceiling assemblies is risky because of the risk of moisture accumulation in the cold roof sheathing.
2. Your roof assembly violates most building codes. (See, for example, the requirements of Section R806.5 of the 2012 IRC.)
For more information on code-approved ways to insulate a cathedral ceiling, see How to Build an Insulated Cathedral Ceiling.
Straw : Cellulose : Wood
Thanks for this blog, Martin. If anyone is interested in learning more about this topic, and you live near Oakland, California, you may want to attend this event: http://www.passivehousecal.org/event/feb-phca-mtg-straw-cellulose-wood
Passive House California (PHCA) is collaborating with Architects, Designers and Planners for Social Responsibility (ADPSR), California Straw Builders Association (CASBA), Ecological Building Network (EBN) and Pankow Blue Construction to focus on three practitioners all using natural materials for hight performance assemblies. These cover houses, offices and mid-rise towers. It should be highly informative.
Responses to comments will be delayed
I'll be on vacation next week (Jan. 12 -16), so responses to any comments posted here (or on other blogs) will be delayed. I'll get around to answering questions, though, when I return to my desk.
Response to Martin & Randy George
Read the exceptions in section R806.5, in particular:
"Exception: The minimum net free ventilation area shall be 1/300 of the vented space provided one or more of the following conditions are met:
" 1. In Climate Zones 6, 7 and 8, a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling."
The key is what the ACTUAL vapor retardency of the Siga is during the wintertime. If it's 1 perm or less in an ASTM E96 dry cup test it would in fact meet the letter of the code. Other membrane type smart vapor retarders (say, MemBrain http://www.naturalspacesdomes.com/dome_store/dome_insulation_systems/images/Membrain3.jpg ) need to have the proximate air to the membrane under 35% RH to duck under 1-perm, which means you probably have to be pretty meticulous about not letting the interior humidity rise too much which can be an issue in very tight homes.
The cellulose will buffer quite a bit of moisture, sharing the moisture burden with the sheathing. While that isn't a solution on it's own in climate zone 6 (central VT), in combination with a smart vapor retarder it gives you a bit of margin.
In the spring once the average daily temps start running 40F it might be worth testing the moisture content of the roof deck in a couple places on the north (the shadiest/coolest and therefore dampest section of roof deck) to see just how much moisture accumulation you're getting into the roof sheathing. It's fine to poke through the Ice & Water Shield for the measurement, since the stuff is "self healing", and will seal punctures over time.
My Roof
Thanks Martin and Dana... I think I can still sleep at night. The critical piece here is the vapor retarder. I'm going to try to find out what the perm rating is for Siga in winter. I'm pretty meticulous about keeping relative humidity between 35 and 40% in the winter. According to what you say, that might be a little too much. I was going over 35 for comfort, but that might not be a good idea with this roof. I've always wondered how much moisture makes it through the sheetrock. If it's 40% outside the sheetrock, maybe the proximate air to the membrane is less? I like the idea of testing the moisture of the sheathing in the spring. That's good to know that I can poke a hole in the Ice and Water shield, but to get to that I have to get through the standing seam.
not Larsen trusses
The assembly in the photo is not really a truss, its more of a series of stacked boxes. A truss would not be needed in this application anyway as the building structure is already framed and these are simply to hold the insulation in place. In fact for DP cell install, this setup is preferable since it create lateral pressure boundaries as opposed to Larsen trusses which have a problem getting the resistance for DP cell.
Foamglas unvented cathedral and basement walls
You mention Foamglas for under slabs, but don't mention it as an alternative to foam for an unvented cathedral ceiling (top half in condensation zone, with fluffy insulation below it), or for basement walls. It seems to me that it would be ideal (in fact the only suitable non-foam method) for those situations, since it is not vapor permeable.
Exterior below grade mineral wool
There seems to be some disagreement on whether mineral wool board can replace foam in below grade applications. Martin doesn't mention it it and implicitly dismisses it: "It’s hard to come up with an affordable foam-free way to insulate slabs on grade or basement slabs." Dana cites rumors of compression over the long term.
But BuildingGreen.com and Roxul disagree. A couple of months ago, in an article titled "Move Over, Foam:Sub Slab Mineral Wool is Here," Alex Wilson wrote that Roxul had approved its ComfortBoard IS product for use under slabs, citing engineering data, research, and a number of successful uses of it in those situations.
http://www2.buildinggreen.com/article/move-over-foam-sub-slab-mineral-wool-here
Roxul's marketing cites a report from 2009 by the Danish Technological Institute on long term compressive deformation that would seem to contradict Dana's concerns:
http://www.roxul.com/files/RX-NA_EN/pdf/Brochures%20and%20Sell%20Sheets/Residential/DrainBoard.pdf
Thoughts?
Thoughts?
I really hope they are right.
Vapor closed exterior - foam free roof w smart vapor retarders
Since the foam free roof proposed by Randy, has a vapor closed roof underlayment (ice & water), the airbarrier on it's interior should indeed be smart/vapor variable. Siga majpell has a fixed perm rating of 0.68perms and while being a class II vapor retarder in winter. It will also be a class II vapor retarder at 0.68 perms in summer - so not variable or smart retarder, a fixed one like siga's specs show - Any construction humidity or moisture infiltration by unforseen airleaks, won't easily diffuse back inward in summer - and will ping-pong there - which can cause rot/mold.
Under a metal roof, which can be easily vented, a vapor permeable roof underlayment like ProClima's SOLITEX membrane would allow the roof also to dry outward and prevent these issues (and be code compliant). However many contractors like ice&water or asphalt shingles, but still would like a foam free roof and a vapor variable retarder can accomodate that safely when installed properly/airtight.
membrain is as Dana points out is a vapor variable retarder, but it does open up rather early - which could lead to humidity diffusion into the assembly in winter. ProClima's INTELLO that my firm http://www.foursevenfive.com distributes stays below 1 perm longer - it surpasses 1 perm after 60%RH and at that time rapidly opens up - see image) - to allow inward vapor drive to dry out any unforseen humidity in summer. Leading to durable, foam free roofs and walls with drying reserves that are larger than the moisture stresses from construction.
Rainscreen Roof?
How would this cathedral roof work? Insulated 2"x10" rafters, taped plywood, 2"x2"s that create a 11/2" vented air space, then another layer of plywood, topped with felt and roofing. The sheathing would get cold but it would have a good chance to dry, similar to rainscreen walls.
I'm in a moderate maritime climate by the way. Thanks.
GUTEX for unvented roof
Another product that supposedly works well for an unvented roof is a wood fiberboard insulation material called GUTEX that 475 sells. It's a highly vapor permeable insulating sheathing, and appears to reduce a lot of the risk associated with dense packing roof rafters.
They illustrate how it can work in a roof here:
http://foursevenfive.com/alford-ma-gutex-roof-makes-sustainable-high-performance/
Dana's comment
Dana - You seem to suggest that randy's unvented roof would meet code with the correct vapor retarder, but the exception you note is for a vented roof (it just reduces the amount of ventilation area). Am i missing something in the code?
moisture checking methods
O.k., I think the various comments that have been made about my roof have made me concerned enough to do a moisture check this coming spring. I'd like to hear about any methods people have of doing this. It has been mentioned that I can pierce the Ice and Water Shield without damaging it, but I still have to get through the standing seam roofing. And once I'm into the Advantech I have to get all the way through that to the inside. I've heard of people piercing the sheetrock and the vapor barrier with a very long moisture probe and going all the way to the sheathing that way. That sounds pretty tough and risky too. Anyone care to comment on how they would check he moisture level on my assembly?
Roof insulation
The platinum standard here in Flagstaff, AZ (Zone 5) is Willow Bend Environmental Education Center. They use ⅛ cord of wood per year and now AC. R50 blown-in fiberglass with energy leg scissor trusses in the vented attic. This performance has held for 14 years. It is a building that is occupied 6 days a week. Temps have never fallen below 55 and normally are at 65 since it is a passive solar building with ⅓ of the south in trombe wall and the rest in glass. Straw bales in the N, E and W walls. We did use rigid foam under slab (4' in from stem) and down the exterior of the stem to top of footing.
Been there, done it, have the award
Excellent thoughts Martin. When we built our Italian CasaClima Gold house, we used many of the techniques that you mention. Have a look (www.vettabuilding.com). We are also starting a new project in Stratford, Ontario that will use many of these systems.
Our cozy cottage
Thanks for this article Martin. It answers many if not all the questions that arise when going down this path. My wife Joyce and I were very uneasy about the use of foam to insulate our new home as had been recommended by other folks we talked to. Thanks to lots of good information from you and other forum participants we came close to the foam-free house.
The only foam we used was for the basement wall and slab insulation, slab on grade insulation (the house has a basement 1/3 and slab on grade 2/3) and spray foam to seal around window and door openings.
I'm happy to say that we are snug as a bug in a rug and are very pleased with the choices we made. Here they are...
Basement slab: Tu-Tuff poly vapor barrier, 2" XPS foam insulation, 4" concrete
Basement walls: 10" thick using Thermomass 4/2/4 insulation system...4" concrete/2" XPS foam insulation/4" concrete.
Slab-on-grade: Tu-Tuff poly vapor barrier, 4" XPS foam insulation (2 alternating layers of 2"), 4" concrete
Slab-on-grade walls: 2" XPS foam insulation inside walls
Above-grade walls: Double 2x4 walls with dense pack cellulose insulation, 10" total thickness
Vented cathedral ceiling: 2x12 rafters with strips of 1" XPS foam insulation on inside rafter edge, 1 1/2" vent space under sheathing and dense pack cellulose insulation.
Response to Dana Dorsett (Comment #8)
Dana,
I agree with Jeff Stern (Comment #17) about code requirements for a ventilated air gap above air-permeable insulation when air-permeable insulation is the only type of insulation used.
Although your comment cites an "exception in section R806.5," the exception you quote is actually found in section R806.2, not R806.5.
The way I read R806.5, there is no wiggle room on this issue, other than the usual (blanket) wiggle room provided by the fact that a local inspector can approve whatever he or she wants.
Response to Randy George (Comment #18)
Randy,
I think that the easiest way to check the moisture content of your roof sheathing (at this point, since you didn't embed monitors in the sheathing when the house was under construction) is to use a moisture meter with very long probes. This method (and caveats about false readings) is discussed in the following two articles:
Dense-Packed Cellulose and a Wrong-Side Vapor Barrier
An Affordable Zero-Energy House in Massachusetts
Response to Bryan Shephard (Comment #15)
Bryan,
Your suggested roof assembly will work, as long as you pay attention to airtightness -- especially at the level of the finish ceiling material (usually drywall). No recessed can lights allowed!
Your method is one of the methods I discuss in my article, How to Build an Insulated Cathedral Ceiling.
Sturdy and insulated
In the coastal areas of the PNW, moisture is a huge problem. Sometimes the rain is blasted at the house by 100+ MPH winds. The older houses framed with notched in diagonals and sheathed in Celutex have far fewer problems with rot than the older ones with more conventional construction. They do tend to sway in the wind.
But newer houses must withstand an earthquake of 9.2 or something like that. And winds of 130 MPH.
Making a house dry, bomber strong, and highly insulated is a lot to ask.
I looked at some of the links including the Larson Truss one. I wonder what those wiser than I would think of the following: frame and sheathe the house to earthquake and wind load standards, cover with a vapor barrier, and install Larson trusses. But then sheathe that exterior surface with Celutex for its breathability. That might be a good way to do it in any case where you are concerned about condensation on the inside of that outer layer of sheathing.
Response to Brian Godfrey
Brian,
You aren't the only builder who prefers to specify a vapor-permeable exterior sheathing like fiberboard. The main disadvantage of fiberboard sheathing is its tendency to belly outward when cavities are filled with blown-in insulation.
For more information on vapor-permeable wall sheathing, see these two articles:
How Risky Is Cold OSB Wall Sheathing?
The Klingenberg Wall
Brian
Faced with the same problems I have been thinking along the same lines as you propose. Build the house to meet the BC seismic code with the required braced wall panels or engineered shear walls. Then add some variation of exterior framing: A second wall to make make a double wall, Larsen trusses or exterior foam. I'm not sure fibreboard is a great choice as it is hard to work with and offers a poor nailing base for siding. Perhaps eliminating the exterior sheathing is the way to go. Although I've still to do a project that goes that route, some of the projects featured here seem to have successfully done so.
Foam-Free Unvented Cathedral Ceiling/Roof
Could Roxul Comfortboard IS be substituted for rigid foam above the roof sheathing in an un-vented cathedral roof assembly? I live in zone 5 so 5" of Roxul (R-20) would be sufficient to keep the sheathihg warm enough to prevent condensation. 2 x 12 joist bays below the sheathing (11.5" actual with no vent space) would be filled with dense pack celluose for an additoinal R-40. Interior would be finished with drywall.
Response to Fred Williams
Fred,
From a building science standpoint, installing mineral wool on the exterior side of the roof sheathing of a sloped roof makes sense. It should work.
That said, it's almost never done in the U.S., so builders who attempt it can expect resistance and skepticism from roofers and building officials.
Here is a link to a Q&A thread where this approach was discussed: Mineral wool roof. On that thread, I posted these comments:
"While exterior mineral wool insulation is routinely used for flat (low-slope) roofs, especially in Europe, its use on steep roofs is unusual. You will be experimenting. ... I recently spoke to [building scientist] John Straube about his building project using mineral wool on the exterior side of a sloped roof assembly, and he said everything went fine, and that the roof didn't feel squishy once the second layer of plywood was installed. (Of course, John wasn't doing the actual installation -- he was directing a contractor.)"
Response to Martin Holladay
Martin,
Thanks for your response and the link to the Mineral Wool Roof thread. That thread led to another thread titled Using Open-Web Trusses as Rafters for Superinsulated Roofs. I'm wondering if an open-web truss would substantially reduce thermal bridging through the structure and mitigate the need for any foam or mineral wool whatsoever? Do you forsee any problems with using a 20" deep open web parallel chord truss with a baffle at the top chord (1.5") for ventilation, filled with 18.5" of dense pack cellulose (R-60), and finished with sheathing, underlayment, and asphalt shingles on the top, and drywall on the bottom? The total thickness of the assembly would be slightly more than the 2x12 plus mineral wool combination, but I assume much easier to build and cheaper.
Response to Fred Williams
Fred,
Q. "I'm wondering if an open-web truss would substantially reduce thermal bridging through the structure and mitigate the need for any foam or mineral wool whatsoever?"
A. The use of a deep open-web truss will substantially reduce thermal bridging through the structure, as Alex Wilson explained in the article you are talking about (Using Open-Web Trusses as Rafters for Superinsulated Roofs).
There never was a "need" for exterior mineral wool. That was simply a choice you considered. Every type of roof assembly you can think of has advantages and disadvantages.
Here's how I would analyze what an insulated roof assembly "needs": If the roof is unvented, it needs exterior insulation (which is usually rigid foam). If the roof is vented, it doesn't need exterior insulation. Remember, however, that not every roof can be vented. If your roof has valleys, hips, dormers, or skylights, it's not a good candidate for the vented approach.
Q. "Do you foresee any problems with using a 20" deep open web parallel chord truss with a baffle at the top chord (1.5") for ventilation, filled with 18.5" of dense pack cellulose (R-60), and finished with sheathing, underlayment, and asphalt shingles on the top, and drywall on the bottom?"
A. The assembly you are talking about (similar to or identical to the assembly described by Alex Wilson in his article) will work fine. Do I foresee any problems? Well, your framers may be unhappy about using open-web trusses as rafters, because they will probably be unfamiliar with the methods required to attach these trusses at the ridge, and they will probably be unfamiliar with blocking details. As I said, every approach has advantages and disadvantages.
The entire conversation seems to assume wood stud framing. But another entirely different approach would be to build with autoclaved aerated concrete. Both walls and roofs can be built from it and it would open up many more options. It's not my intention to go into them all here. I just wanted to point out that other avenue of exploration.
Antonio,
You're absolutely right that above-grade walls can be built with autoclaved aerated concrete.
According to my limited research, however, autoclaved aerated concrete can't be used for basement walls or crawlspace walls. See, for example, the detail below, taken from the web page of a manufacturer of autoclaved aerated concrete blocks in Florida (Aercon AAC).
Assuming my understanding is correct, an autoclaved aerated concrete wall still needs a concrete foundation (a slab, basement, or crawlspace foundation), so the issue of foundation insulation still needs to be solved.
Finally, autoclaved aerated concrete walls have two major hurdles, at least in the U.S.: relatively low R-values and problems with availability. Few U.S. builders live near a manufacturer of autoclaved aerated concrete -- and the farther a job site is from the manufacturing facility, the more likely that transportation costs will be prohibitive. For more information, see "Does Autoclaved Aerated Concrete Make Sense?"
Thanks, Martin, for the helpful article.
Could one of these foam-free assemblies be adapted for a timber frame roof with exposed rafters, and cedar plank ceiling? This wood has already been milled from our property so we’re compelled to use it. Ideally we could avoid the plywood sandwich, perhaps the cedar could replace the first plywood layer? Though I can’t imagine it would be easy to air seal this layer. Thanks again!
Adam
Adam,
There are at least two ways to install insulation above the type of ceiling you describe:
(1) You can choose an unvented assembly, which would usually involve the installation (above the cedar boards) of a high-quality air barrier membrane, followed by an adequately thick layer (or layers) of rigid foam insulation -- or, conceivably, if you don't like rigid foam, by an adequately thick layer of dense semi-rigid mineral wool insulation -- followed by a layer of OSB or plywood roof sheathing, roofing underlayment, and your choice of roofing. For more information on this approach, see "How to Install Rigid Foam On Top of Roof Sheathing."
(2) You can install a vented roof assembly, which would usually include the installation (above the cedar boards) of a high-quality air-barrier membrane, adequately deep rafters installed 16 inches on center or 24 inches on center, the insulation of your choice between the rafters, a site-built ventilation channel between the top of the insulation and the underside of the roof sheathing, and OSB or plywood roof sheathing, followed by roofing underlayment and the roofing of your choice. Note that the ventilation channel needs to be connected to soffit vents at the bottom of the roof assembly and a ridge vent at the top of the roof assembly.
"While it’s fairly common to install semi-rigid mineral wool insulation above roof sheathing, the method is usually restricted to low-slope (flat) roofs. Using this method for sloped roofs would be considered experimental." Now that it's 2021 I don't think this is considered experimental anymore? And what would be the issue anyway, compared to a flat roof?
Qofmiwok,
I agree that such an approach is no longer experimental. It's simply rare.
Log in or become a member to post a comment.
Sign up Log in