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Paul DeGroot is an architect in Austin, Texas. His projects tend to be on the modest side in a city that has its fair share of high-end custom homes. He’s not building affordable homes, but neither are his projects at the upper end of the market. That means he is always working to balance his clients’ budgets and to make sure that he’s delivering the features they are after while still designing a quality building.
In a market where the norm is to fill the stud bays with fiberglass batts, sheath the house with OSB, install a lumberyard-branded housewrap, and nail the siding directly to the wall, Paul tries to do a little better. When the budget allows, he specs mineral-wool cavity insulation, a ventilated rainscreen behind the siding, and ZIP System R-sheathing.
Austin’s building codes call for wall insulation minimums of R-15 cavity insulation or R-13 cavity insulation plus R-2 continuous insulation. Paul recognizes that the latter is a better option from a durability perspective but is wary of builders’ ability to get the details right when installing exterior rigid foam outside of the sheathing. It just isn’t common enough yet. This is one of the reasons he specs ZIP R-sheathing when the budget allows.
ZIP R-sheathing panels have the insulation on the inside—installed, the insulation is in contact with the studs. On the outside, they have ZIP System’s proprietary water-resistive barrier (WRB) over a structural OSB panel. In this way, the panels provide a thermal break with continuous insulation, but windows and doors can be installed in plane with the sheathing with no need for the fussy flashing details often associated with exterior rigid-foam insulation. That gives Paul some confidence.
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28 Comments
I chose to use the R12 Zip-R on my house in the Denver area. Because ironically, I couldn't find a contractor that was willing to use exterior rigid insulation. I had originally designed it with (2) 2" layers of rigid foam over OSB with a nailing strip / rain screen over the foam. But, attaching the exterior sheathing really confused them since they had never seen it or done it before. Somehow the Zip-R sheathing gave them a higher comfort level. I think the only thing I would have changed is to use the liquid flashing in lieu of the tape. The Structural Engineer made us screw the Zip-R since it was so thick. The screw heads were not flush to the OSB and we wound up having double widths of tape to cover the screw bumps. The liquid flashing would have made that application much easier.
Thanks for sharing your experience Lawrence. I heard more than once while researching this that the panels shouldn't be screwed. Do you know what type and length of screws were specified? I'd be curious to ask the folks at Huber about that.
As far as I can tell, Huber doesn't specify any screws as an option.
We found using some screws was helpful to pull certain sheets tight/flush with adjoining sheets in odd places. We would use the screws as clamps and then nail off the piece. Air nails alone didn't always pull the sheet as tight as you would like.
We are concerned about not having over driven fasteners, so we had the gun depth control set for minimum penetration which sometimes left the sheet gapped from the framing.
We did ask Huber and they deferred to the Structural Engineer recommendations. We used the 4” long Strong-Drive® SDWS TIMBER Screw with a 6/12 screwing pattern (6” on edges and 12” in the field). We also had difficulty getting enough of these screws and for a while had to buy a 4 1/2” HeadLok Timber screw. Both of these screws had flat heads and were self tapping. The framer doubled the studs at all the joists so that the screws at the panel joints did not have to share a stud.
One of the issues that came up was the length of nails required for the 2 1/2” panels was a 4 1/2” nail (as I recall) which is longer than any nails that we could find — other than some out of stock product from a sketchy web site. And, not many nail guns can take a 4 1/2” nail.
Since you have to buy the Zip-R panels in a palette, we would up with almost a full palette extra. So, we used that to sheath the wall between the garage and living spaces in the house. We did use 4” nails in a wall in this application. But, the Structural Engineer and Huber both agreed since it was an interior, non-structural / non-shear application, it would be okay. Our framer still had to buy a new gun for that.
Having said all that, it appears that Huber has changed their instructions on the Zip-R 12 panels. They now recommend a 4” nail at a 4/12 nailing pattern and the Zip-R 12 panels should not be used for shear. As I recall when I was building, they were recommending 4 1/2” nails at a 3/12 nailing pattern. The Structural Engineer was very confident in the shear capability of the screwed Zip-R panels. We have been here for about 18 months after completion through numerous high wind events and the house seems pretty solid.
I guess, in the future, I might use diagonal metal strapping or 1x4 Let-in bracing for shear and just nail the Zip-R 12 panels. Had I known the screws were going to be such a nuisance, I would have gone this route in the first place.
How much concern should there be about the diminishing R-value of the Polyisocyanurate at lower temperatures on walls in climate zone 6?
>"How much concern should there be about the diminishing R-value of the Polyisocyanurate at lower temperatures on walls in climate zone 6?"
There would be very little to be concerned about from a seasonal energy use or peak load point of view, but as least SOME concern from a dew point control perspective if designing close to the margin.
In Zone 6 the IRC prescribes a minimum of R7.5 on 2x4 framing (with a presumptive R13 cavity fill), R11.25 on 2x6 framing (with presumptive R20 cavity fill). The R9.6 ZIP-R may underperform the R7.5 during Polar Vortex disturbance cold snaps, and the R12.6 will likely underperform the R11.25 in the cooler weeks of even normal winter weather.
It's possible to mitigate against polyiso derating that may be too close to the margin by using cellulose or denim cavity fill, either of which has substantial hygric buffering capacity, &/or installing a "smart" vapor retarder such as Intello Plus or Certainteed MemBrain behind the interior side gypsum board.
Is there a way to know for certain what the R-value of polyiso drops to at various temperatures? What I am thinking to do for a Climate Zone 6 house is 2x6 framing with R20 cellulose in the stud bays, then OSB sheathing, and then Atlas EnergyShield 2" polyiso foam (rated by Atlas at R-13.1 at 75° F) taped to act as a WRB, rainscreen strips, and wood siding. But I want to make sure that I am not too close to the margin in colder weather. Is this a safe assembly? Does the foil facing on the EnergyShield change the R-value differential?
In addition, with this buildup, would it be bad to add Tyvek between the OSB and EnergyShield? Any benefit to that? Or is that going to sandwich the foam in a bad way?
>"Is there a way to know for certain what the R-value of polyiso drops to at various temperatures? "
No. It varies by manufacturer, and they don't publish that data. Dow's Thermax has allegedly broken the paradigm and needs no derating.
Note, the stated temp of the ASTM C518 test is the AVERAGE temp through the foam layer, not the temperature of the cold or hot side. A product labeled R13 @ 75F is likely to hav been tested with 100F on the hot side, 50F on the cold side.
When there is an insulated wall between the polyiso and indoors it lowers the average wintertime temp at the warm side of the foam layer, but the R-value of that insulated wall changes the calculation too. If the house is always 70F and your average outdoor temp in January is +10F (the cold edge of zone 6) for a 60F delta-T, with R20 in the cavities and a presumptive R10-ish derating for the 2" polyiso the warm side of the foam will average about 30F, and the mean temp through the foam layer would be about 20F.
A worst-case product used in Dow Thermax marketing fluff can be found in Figure 2 of this document:
http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_09b4/0901b803809b49c0.pdf?filepath=styrofoam/pdfs/noreg/179-00263.pdf?filepath=styrofoam/pdfs/noreg/179-00263.pdf&fromPage=GetDoc
Note that the 75F mean temp rating of that (probably 2lb roofing foam) product is a pretty crummy R5.2/inch, which derates to about R4.3/inch at a mean temp of 20F. Assuming the R13 Atlas EnergyShield product scales similarly (which it might, but who knows?) one would expect...
R13 x (R4.3/R5.2)= R10.75
...at a mean temp of 20F through the foam.
It's pretty safe to assume at least R 10.75 average performance even in January in your stackup, higher in December & February, and it's the seasonal average that matters from a moisture accumulation point of view. A foil facer next to a 3/4" rainscreen gap adds about R1 to the performance. The cellulose cavity fill can also buffer a very substantial amount of wintertime moisture without damage or loss of function, which makes it less risky when designing near the margin on exterior foam.
The mean January temps in much of zone 6B are north of 20F, which would move the derating curve adjustment into the R11s. In say, Helena MT just eyeballing it the mean outdoor temp for the coldest weeks on the cusp of December/January is about 22F, with a seasonal mean temp of north of 25F for the December-February period:
https://weatherspark.com/m/2550/1/Average-Weather-in-January-in-Helena-Montana-United-States#Sections-Temperature
Thanks for the info. Basically, it sounds like in your opinion my proposed wall assembly would likely be safe in Helena MT as long as the EnergyShield product derates similarly to the polyiso in that chart, right? How about any benefits or issues with adding Tyvek between the OSB and the EnergyShield? Or is taping the EnergyShield as the lone WRB safer?
Tried ZIP R recently. Overall thought it was a good product. I have used un-insulated Zip sheathing and been very happy with it.
Really dislike the fact that sheets delaminate so easily. For the cost of this product, a better glue should be used.
I am concerned about using this ZIP R with porous insulation in the cavity. Any air migration in the cavity could deposit moisture at the cold joints in the ZIP R. This wetting action could cause problems, but I don't know that it does. A rain screen system would help this moisture move outward.
I have a reservations about the foam being "on the wrong side" of the sheathing, but it seems like its been working ok so far.
Nailing is tricky. If the air gun doesn't set the nail perfectly, you have to resist the temptation to use a hammer at the panel perimeter. Using a hammer compresses the foam rather than just sinking the nail. Nails in the field can be set with a hammer as needed.
We used the R-9 2 inch sheathing and did have to buy a new nailer to shoot 3 1/2" nails as all our nailers would only handle 3 1/4".
Zip tape is great, but it requires fastidious installation to avoid a wrinkle that can funnel water into the panel joint. We applied a bead of ZIP Liquid flash on the high side of each horizontal tape joint to keep water out in such instances. We noticed the liquid flash sometimes seemed to have poor adhesion to the seam tape, but since this was being used in a "belt and suspenders" approach we felt the chance of both the Liquid Flash and the seam tape developing an issue in the same location was minimal. We also apply liquid flash over all the nail holes.
Liquid Flash took about a week to harden in cold weather (highs in the 40s). If the sheathing was at all damp, (even just a foggy day) we found the liquid flash would not stick and just rolled off when we tried to tool it.
ZIP stretch tape, (Different than seam tape) is an amazing product.
We had planned to use liquid flash for the window openings but it would have taken a week or more for the liquid flash to cure. We framed our window R/Os with a sloped sill and used the stretch tape in the corners and the seam tape for the straight areas. No dry times, and we eliminated the PVC pan flashing we typically use in windows.
It seemed like it would be difficult to get an accurate window opening cut into 2 inch thick sheathing with a saw. For the first time ever, we used a router to cut window openings and had a shop vac on the building interior. The window openings came out perfect, and the foam was clean and smooth which allows the tape to stick to it nicely.
Great Feedback. Thanks David.
We used the R6 panels on our current build with the students and it worked great. This was the first time using it as well and we used the cordless Hetabo HTP framing nailers that have depth control and had zero issues with over driving or squeezing the foam. Definitely remember to adjust for the extra thickness when laying out framing from the corner. Very happy with it and got great results.
One thing that doesn't jump out to Zip R users in high wind and seismic areas is that the prescriptive nailing pattern and allowable shear capacity is that the tables are based on Dfir framing lumber. It used to be just noted in a footnote but now Huber puts that info in the table titles (noting that they are relevant to Dfir/Larch).
So users who follow the IRC prescriptive method have to use Dfir since the table have any adjustment factor. Users following the engineering method would have to make an adjustment for the specific gravity of the lumber they're framing walls with. That would translate into reduced shear capacity for most framing lumber except SYP which has a higher specific gravity than Dfir.
For example, using Zip R12 panels over SPF framing would reduce the shear capacity from 215 lb-lf to 195 lb-lf - about a 10% reduction.
While most of north america wouldn't have to worry much about such a reduction, those building in high wind zones and seismic zones have to pay close attention to the limitations.
First off, I’m not in the construction field, I am a buyer just trying to learn. I’m in zone 3 and the IRC states that the wall insulation needs to be R20 or R13 + 5. If the wall is built with Zip-R6 and a standard R13 batt would that meet code? The way I am understanding it, Zip-R is not really exterior insulation since it’s all on the inside of the sheathing so I would only have a R19 wall (with a thermal break bonus) and not meet code. Is Zip-R more like a flash-n-batt than a true exterior insulation?
Hi David.
What meets code is really up to one person, the building official where the house is being built.
Using ZIP R-sheathing is not like continuous exterior insulation because the OSB sheathing with the ZIP R-is outbound of the insulation and not kept warm by the insulation. But it is no more like a flash-and-batt wall, which doesn't have continuous insulation and does nothing to mitigate thermal bridging. Best to think of it as it's own type of assembly.
The first condensing surface with ZIP R-sheathing is the inside face of the rigid foam layer. It's best to have high enough R-value (thick enough ZIP R) to keep that surface above the dew point temperature. I agree with Allen Sealock: It's best to use the R-value requirements for continuous insulation found in table R702.7.1 of the 2018 IRC, which are a bit higher than those found in the energy code. It's also wise to have a ventilated rainscreen, so the ZIP R OSB layer can easily dry to the exterior (and for other reasons).
Brian,
I would argue that ZIP - R is continuous insulation, and does prevent thermal bridging. It is similar to flash and batt but the studs are not directly in contact with the exterior sheathing. Just like using Hunter Panel or any other nailbase system. Use infrared and it will disprove your statement that those systems "does nothing to mitigate thermal bridging".
I totally agree with you on the ventilated rainscreen and to keep enough R value to prevent condensing at the Zip Layer.
Brad, I think you are misreading Brian's post.
He says that zip r is not like continuous exterior insulation in that the sheathing component is outboard of the CI.
And that it's not like flash n batt since flash and batt doesn't break thermal bridges (meaning zip r does).
This may have been agreement: having enough r value is to prevent condensation at the inner face of the foam of the zip, not the osb of the zip.
Tyler,
I fully understand that Brian is saying Zip-R is NOT exterior insulation. I agree with him on all points actually, except that Zip-R certainly DOES mitigate thermal bridging. IR proves it. Obviously the thicker the Zip-R the better it works at doing that.
The days where we stop adding 1/4", 1/2", 1" insulation to the exterior or even thin Zip-R can't come soon enough in my opinion.
Hi Brad,
Sorry my comment above was confusing, but if you re-read it, I think you'll see that me, you, and Tyler are actually in agreement.
To clarify, ZIP R sheathing does qualify as continuous insulation because the layer of polyiso is not interrupted by studs. Therefore, both exterior insulation (a layer of mineral wool installed outside of the sheathing, for example) and ZIP R sheathing mitigate thermal bridging through the wall framing to the degree that their R-value provides.
A common flash-and-batt assembly, however, is a layer of closed-cell spray foam insulation installed against the sheathing followed by batt (or blown) insulation to fill the remainder of the wall cavity. Because this is all installed within the stud bays, it does not mitigate thermal bridging.
Here's a photo, from Fine Homebuilding, of a flash-and-batt installation.
“[Deleted]”
Just a quick note on shear walls, I wanted to use Zip R6 for my design, which is a homeowner remodel as it will be easier for me to install and detail. I have existing 2x4 construction so the R6 allows me to meet code for zone 4 marine here in the Seattle area. I worked with my engineer who had never used it before and he specified that I just install additional plywood shear panels under the Zip R where we needed more shear strength due to the seismic requirements.
I have not got to the point of installing it yet so thanks for all the tips. I want to install hardieplank siding over it and am considering using Paslode's hardinails and Slicker classic rainscreen, which should be thin enough for me to use the shorter nails and not have to worry about hitting studs.
As some have mentioned, it's somewhat expensive on the material/tool side but since my labor is cheap (ha) and time is not really an issue for me, I have less restrictions on that side.
Postscript on rain screen. Been going back and forth on whether it is really necessary with my design. Hardie says it has never been required at all with hardieplank, but there seems to be a lot of strong opinion out there that you have one. So I think I'm going to skip the rainscreen and just nail directly to the Zip board with the hardinails. After looking at it a bit I think the Slicker rainscreen is too thick so the only other rain screen option with the hardienails would be to use thin drainable house wrap, and I'm thinking that would be over kill as the zip finish is probably just rough enough to drain itself. I know this is may be somewhat off topic but If someone has a different opinion, I'd be interested in hearing it.
Ron,
I would never build in Seattle without a rain-screen, especially without a separate WRB, and just relying on the wax finished Zip. I agree though that if the alternative were drainable ho0use-wrap, it isn't worth doing.
Ron,
Zip finish is rough enough once it swells. Definitely use a rain screen......Rainscreen will always make siding last longer and paint too.
Malcolm, did you write "hoouse-wrap" because you are Canadian?
:-)
Rick,
Ha!
I can't type, so my posts are riddled with those sorts of errors.
Vapor retarder confusion
Climate zone 5 (NW Detroit suburbs)
Brick
2” zip r sheathing
2x6 framing
Fiberglass bibs
Do I need some level of vapor retarding inside or do I want it open to vapor?
I wasn’t planning on a smart vapor barrier...
I’m trying to decide if I need some level of vapor retarding paint (or what to avoid).
Thanks
Alan
Curious, why the double bottom plate in the 7” bottom plate w 11/2”zip and 2x6 studs.? Thx
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