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Community and Q&A

Feedback on this Wall Assembly Plan

Joe_PA | Posted in General Questions on

I am building a ~1,200 sqft addition on a 1914 house in Zone 5, have been reading extensively about exterior walls and control layers on GBA and BSC, and am still befuddled.  I am hoping that this Community Forum can provide some guidance.

My architect has specified the following from the outside and moving in:
1) Exterior cladding (with or without drain mat depending on choice of materials)
2) 1″ continuous EPS
3) WRB
4) Sheathing (let’s say 1/2″ plywood)
5) 2×6 Wall
6) Cavity Insulation – Flash with closed cell foam and fiberglass batts
7) 5/8″ gypsum (current house has lath/plaster walls, so we’re looking for a solid feel in the addition, we may go with Type x, for added density.)

My contractor is pushing hard to replace the sheathing/WRB in this design with Zip Panels and tape (Zip 1.0).

In addition to any comments you may offer, my questions:
a) If the closed cell foam is vapor barrier, then am I correct to assume the WRB would need to be vapor permeable to avoid a double barrier (or is Lstiburek’s Double Play article from 2019 tell me it is fine….  His figure 4 starts looking like my situation, but only uses closed cell in the cavity)?

2) Lstiburek’s article also suggests a gap between the foam and the WRB.  Would you recommend something like Drainvent, Hydrogap, or another product?

3)How do you feel about this same wall, but using 7/16″ or 1/2″ Zip System?  I have a predisposition against commodity OSB.  I understand that Zip’s materials are different and may not suffer the same problems.  Nonetheless, my perception is that plywood is more durable –  can handle more water, dry more quickly, and return to the same shape.  I also am concerned about the Zip tape system and the potential for water ingress at the seams (a shingle style wrap seems to use gravity in one’s favor and a better long-term solution).

4) Is this wall design prone to condensation issues in Zone 5?

5) How would you change this wall design to improve it. (for a variety of reasons, let’s assume I’m limited to the 1″ CI on the outside)

Many thanks for looking and sharing!

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  1. Expert Member
    Michael Maines | | #1

    a. WRBs are always vapor permeable--anywhere from 10-ish to 60+ perms, depending on the product. A flash coat of closed cell foam is still slightly vapor permeable. EPS is vapor-open.

    2. I'm not sure what Lstiburek article you're referencing but the ones I'm aware of that promote a gap are for when the exterior insulation is not vapor-permeable and without enough R-value to prevent condensation.

    3. I am totally comfortable with Zip sheathing as long as the seams are properly rolled and there is a rain screen gap so the sheathing is not getting wetter than necessary. It is better than most OSB but it is still OSB and won't stand up well to extensive, repeated wetting, or at least not as well as plywood or board sheathing, so I prefer to use it in conjunction with exterior insulation. (Enough insulation to keep the sheathing above the dewpoint temperature.) It is very common for Zip tape to NOT be properly rolled; it will stick with light hand-pressure but it only works as intended if the acrylic adhesive is under pressure at installation. Rollers are available with a "Z" imprint that lets you confirm it was rolled.

    4. Prescriptively, to be safe from condensation in CZ5, you need R-5 sheathing over a 2x4 wall (insulated to ~R-13) or R-7.5 over a 2x6 wall (insulated to ~R-21). ( That works out to about 26-27% of the R-value on the exterior of the condensing surface, in your case the interior side of the closed cell foam. If you have R-4 EPS and R-13 batts, that leaves space for 2" of spray foam, or about R-11. That leaves a ratio of R-15:R-28, or 54% of the R-value on the outside of the condensing surface, so you should be reasonably safe. If you are planning on other levels of flash and fill, the equation will change.

    5. This seems to me like a complicated assembly, with three different types of insulation and multiple steps required for the exterior insulation and cladding, and the result is not particularly robust in overall R-value because there is so little exterior insulation. I might simplify and reduce my climate impact by eliminating the spray foam and filling the cavity with borate-treated cellulose, which will protect the materials it touches from microbial action, and I'd add an interior variable permeance vapor retarder so very little moisture will get into the assembly.

    1. Joe_PA | | #2

      Michael, (apologies if this reply is duplicative. I drafted a response, and GBA had me sign in again; so I rewrote it.)

      Thank-you for the response and thoughtful comments. For reference, the Lstiburek article is this one: where he writes, "we should have two “drainage gaps”. The first is obvious...immediately behind the cladding. No surprise there. The second? Ah, between the continuous exterior insulation and the “water control layer”. How much of a gap for the second gap? What works is somewhere between 1/32 of an inch and 3/16 of an inch." I have not seen many others discussing a gap between the C.I. and the sheathing (assuming Zip or plywood with a wrap, which is the water control layer, right?)

      In your response to #4 - Your response is reassuring regarding the ratio of insulation outside the condensing surface (the inside edge of the spray foam). I had thought of the condensing surface as the sheathing and not the interior side of the spray foam (which using your example numbers would have only a ratio of R-4/R-28 on the outside of the sheathing). This was one of my concerns about Zip (condensation + OSB) and why I was thinking about plywood and a drain wrap of some variety. Is condensation on the sheathing not an issue because 2" of closed cell spray and/or the gypsum retard/block/control the interior vapor from reaching the sheathing?

      Regarding #5 - I will look into it!

      Thanks, once again. Any further comments and thoughts are welcomed!

      1. Expert Member
        Akos | | #3

        +1 on #5.

        This wall has been built here for a long time (zone 5 and 6) and works even with regular batts and 6 mil poly. Cellulose is definitely the better option if you have local installers.

      2. Expert Member
        Michael Maines | | #4

        Ah, I'm sure I read that one when it came out but I thought you might be referring to the 2013 "Mind the Gap, Eh?" which I recall was more about including a gap if you know the exterior foam insulation won't have enough R-value to prevent condensation. Either way, it sounds like there's not much downside to including a drainable WRB. Last summer at the Westford Building Science Symposium, John Straube presented on drainable WRBs and said that it's amazing how much water will drain from even Tyvek Drainwrap, which is just a crinkly version of regular Tyvek.

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