I’m planning on using a construction method described in “Breaking the Thermal Bridge”, FH#250. This involves adding a foam furring strip combination over 2×6 exterior walls to create a thermal break. In my case, I’m having a 1″ layer of foam sprayed in the stud cavities as a starting point. I then intend to use fiberglass batts. My question has to do with the depth of the combination wall – 5 1/2 studs plus 1″ of foam plus 3/4″ of furring. So the cavity to be filled is 7 1/4″ minus the 1″ of foam or 6 1/4″ of final depth. If I use 5 1/2 fiberglass batts, I end up with 3/4″ of air space between the foam and the fiberglass. Is this acceptable? Any thoughts are appreciated.
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Why not...
...just use 1" of foam sheathing everywhere? I didn't see the FHB article, but it seems to me that 2" (1" sheathing + 1" sprayed) of foam is better than 1". Foam sheathing would involve less labor than the furring. I couldn't swear to it but my guess is that sheet foam is cheaper than sprayed on foam. What am i missing here?