More Q&A Spotlight
A growing number of designers of high-performance homes are spec’ing double-stud walls in cold climates because they like the assembly’s higher insulation R-values and ability to minimize thermal bridging. A typical double-stud wall consists of two stick-framed walls separated by a cavity for insulation—typically blown-in cellulose but fiberglass, mineral wool, or wood-fiber batts are used too. Total wall thickness ranges between 9 and 14 in. The spacing depends on the R-value objective. For example, a 5-in.-deep gap between 2×4 walls provides room for 12 in. of cellulose, yielding an R-value of about 40.
Not all builders are familiar with this wall assembly, although one of the reasons designers like it is because of the standard building materials and methods, which are familiar to anyone accustomed to framing 2×4 or 2×6 walls; notably, this assembly does require custom framing around windows and door openings and other penetrations.
Double-stud walls do present some questions such as those posed by owner-builder “AZVuilt” (AZ) in this Q&A thread. Located in Climate Zone 5, AZ’s primary concern is using Zip System sheathing as the primary air barrier. Zip insulated sheathing includes OSB—a material that is sometimes questioned for cold-climate applications because of the potential for moisture accumulation during the winter months, which could result in anything from mold growth to wood rot.
Specifically, AZ wants to do a variation of Joe Lstiburek’s double-stud assembly, as described here. In Lstiburek’s version, air and vapor control are handled with structural sheathing installed to the interior wall of the cavity rather than on the exterior, which allows the wall to dry to both the outside and inside, minimizing condensation inside the wall. And while Lstiburek suggests OSB as a sheathing option, as mentioned, some pros advise against using OSB in a cold climate, where the sheathing could stay…
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12 Comments
The Minnesota Department of Commerce did some test walls years ago using OSB sheathing. As I recall they were built to code, warm side air barrier, R-19 or R-21 and OSB exterior sheathing. I think Dept. of Commerce was interested in moisture accumulation in the walls, the test walls were in different parts of the state. No idea what the conclusion was, some digging may be in order.
Doug
I'm building a double stud wall with ZipR6 now. It will have a 3/4" ventilated rainscreen, densepack cellulose and a variable interior vapor retarder. I'm not concerned about it. I think it will perform well.
conwaynh85,
I would be a bit concerned about that wall. It has very poor drying capacity to the outside, and the foam probably isn't thick enough to stop condensation on the interior face. Double-stud walls and exterior foam are typically viewed as either-or choices.
https://www.greenbuildingadvisor.com/article/exterior-rigid-foam-on-double-stud-walls-is-a-no-no
Malcolm,
The Zip panel is open to the 3/4" ventilated rain screen and has a lot of drying potential. The foam on the zipr appears to be a class 2 vapor retarder. With a variable perm interior vapor retarder, I don't see that it will have an issue with cellulose in the wall. Having said that, I'm putting an omnisense in the wall to see.
conwaynh85,
What keeps double-wall assemblies safe is complex, but using Zip-R definitely increases the risk. Whether it's enough to tip the wall into experiencing moisture problems I guess is an open question.
The puzzle for me is what it adds? You already have a thermal break in the framing between the two walls, and as gratefulben said, one of the chief merits of double stud walls is that they allow you to end up with any R value you want simply by increasing their width.
Malcolm, I agree on all fronts, but conway is right in that the least-permeable part of Zip R6 is still the OSB. I still would never use that assembly because the foam does slow vapor movement even more than OSB alone, and there is no need for an exterior thermal break when there is already one in the center of a double-stud wall. Or there should be.
As Malcolm mentioned that is a potentially highly problematic combination because of the nearly non existent drying potential to outside. Just make the space between your walls slightly thicker (1.5”) and you’ll have the same performance without a dangerous moisture balance.
I have monitoring data showing there is little to no difference to the seasonal moisture accumulations or drying rate profiles between CDX and Zip. I’m not sure that I share the same concern as Mike about degradation due to annual moisture cycling. Anecdotally, having watched zip that was exposed strictly to cyclical high moisture contents, without UV exposure, the material changes happen predominantly during the first wetting event and are not compounding over time. Sure, CDX performs better but I’m not sure it’s enough to warrant a material change in non high moisture load building types.
gratefulben,
That's interesting. I would expected the CDX to dry faster due to it's higher permeance.
As did I but the data doesn’t support that being reality. What I’ve seen is that when both wall system hit their highest moisture levels at the end of winter the CDX starts to dry a couple (2-5) days before the Zip. In that time frame there is no noticeable additional moisture accumulation in the Zip and both drop back into very safe moisture ranges well before surface temperatures are any where near the range to support biological decay causing organism growth.
I’ve been meaning to write something up about this for a few years now. When I get some free time…
-BB
“[Deleted]”
I've had two sheets of Advantech outdoors for 8+ years, standing as an A-frame. They remained in perfect condition for several years but in the last few years they have started to degrade rapidly. I'm a big fan of Huber and their products, but Zip is still OSB, and has less resin in it than Advantech, and with far more wood surface area than plywood has.
Every wetting cycle, those wood chips expand and contract a little. I would not expect to see problems for at least 15-20 years, and very likely longer than that. But I don't want to be the one responsible for failing sheathing in 50 years.
I made a few mistakes with OSB and SIPs when I was a carpenter 30 years ago that keep me up at night. I'm not one to expect the homes I design to last 500 years but I'd like them to last at least 50 years without problems.
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