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…
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
9 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,
In terms of risk, the list of exterior sheathing goes something like: fiberboard, plywood, OSB or ZIP, Zip-R. Where on that list the risk tips into the wall experiencing moisture problems I guess is an open question, but choosing Zip-R is definitely designing risk into an assembly that doesn't need to be there.
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.
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
Log in or become a member to post a comment.
Sign up Log in