CZ6 Wall Assembly: Minimal exterior continuous vs none?
I’ve read as many articles on various wall setups (including on GBA) as I can. I’m in CZ6 and the most common setup I see (I’m a rater working with many different builders) is a 2×6 wall with batt/BIB cavities and poly inside. I’m aware of the issues / non-need of interior poly and so on. The main question I have is that some builders use 1″ XPS rigid on the exterior while some are also asking about ZIP R. I’ve read the articles about CZ6 needing R11.25 continuous exterior to protect the wall sheathing from condensation/moisture. The reality for the builders I work with is that they’ll never do 3″ EPS on a 2×6 wall. It’s a cost and siding issue for builders who build on volume/cost. That being said, these builders still have interest in making good building science decisions within their cost structure. That leads me to wondering: is doing 1″ XPS or 1 to 1.5″ ZIP R a good recommendation for them versus doing no continuous at all? Or are they better off not isolating the wall sheathing at all and letting it dry better by having zero continuous because being “half way in” via 1″ XPS or 1.5″ ZIP is no-man’s land or is riskier for the integrity of the wall than it’s worth?
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I'm just a hobbyist but in British Columbia, a very common assembly appears to be (from inside to outside):
- Drywall
- Poly vapor barrier
- 2*6 walls with batts
- Sheathing (OSB or plywood)
- WRB (Tyvek or otherwise)
- Rockwool comfortboard
- Furring strips (rain screen)
- Siding (often something like Hardie Board but there is a lot of variety)
The nice thing about that assembly is that it can clearly dry to the outside.
Seth-R,
You can mitigate the risks. These assemblies with 1" of foam are very common in Ontario and Quebec, and they seem to work fine:
https://www.greenbuildingadvisor.com/article/rethinking-the-rules-on-minimum-foam-thickness
Thanks Malcolm. I hadn't found that article yet. More answers, but more ambiguity :P. It seems like in CZ6 I'm settling on safest practice would be R11.25 exterior and no interior poly. Given unlikelihood builders will go for that, the next best, while still "safe," option will be less than R11.25 exterior (1" R5 rigid or ZIP R) and a smart retarder on inside (Membrain) with an interior wall that is airtight, and a home that's not pressurized nor humidified. Thanks again!
Seth,
Yes ideally, although the experience in Eastern Canada is that these walls still work using interior poly, as long as the foam allows some drying (not foil faced).
2021 IRC Table N1102.1.3(R402.1.3) shows R20 +R5ci as an option. I would not use poly vapor barrier on the inside when using impermeable outsulation, but a smart retarder membrain, if needed.