Durable wall assembly?
I am in discussions with a modular factory home builder in my area. Climate zone 7a. I would like to achieve a net zero ready envelope following the Pretty Good House method. For my climate R40 walls looks like a good target.
Their preferred wall assembly for achieving R40 is using 2×6 and 2×4 offset studs with 10” thick walls and fibreglass batts. This alone wouldn’t get to R40, but they mention adding to this assembly outside insulation at 1.5” foam board (R5) or 2.5” foam board (R10) to get to R40 range.
From the articles I’ve read on GBA this assembly sounds risky in my climate zone with the low permeable ESP outside foam insulation ratio to the permeable cavity insulation at 24%. I look forward to reading some informed thoughts on this wall’s durability.
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Replies
bar_king,
Double stud walls we know benefit from a few things to keep the sheathing safe.
- Good drying to the outside (so no exterior foam), a high-perm WRB, and a rain-screen gap.
- A good interior vapour-retarder and air-barrier.
- A cavity insulation that buffers moisture, like blown cellulose.
The one I don't think you can compromise on is is the first. There are a few other options to up the wall's total R-value. The first is to use a permeable product like mineral wool boards for the exterior insulation. You could also add the foam to the interior, or widen the gap between the two stud walls to hold another layer of batts.