I am planning on building a house in Massachusetts. I want to minimze the thickness of the wall so as not to lose too much interior square footage, but I would like to maximize the R value of the wall (target is 40). I have been thinking about using a staggered stud wall with 2″x4″s on 2″x8″ plates. I was thinking about using 3″ of closed cell foam against a zip wall sheathing sytem with either 3 1/2″ of fiberglass batts or blown in cellulose inside. My questions are, 1) does the closed cell foam act as a vapor barrier and if so, is it enough to prevent condensation from forming in the middle of the wall in winter and, 2) does the foam also act as an air barrier and, if not, won’t an air barrier on the warm side create two vapor barriers? Thanks for any help.
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