Exterior/Interior insulation ratios – 2×4 insulation in a 2×6 wall?
I will be using 3″ exterior GPS foam on my house (R 14.5). The walls will be 24″ o.c. 2×6. I want to use Rockwool bc of its properties over fiberglass; however, it would give me an r-23 in the cavity. This would give me an exterior/interior ratio of 38 (total r-value of 37.5); just above the recommended minimum. I was thinking of using 2×4 Rockwool (R 15) (instead of the 2×6). This would give me an exterior/interior ratio of 49 (total r-value of 29.5). I would tac in place using furring strips.
I’m not sure which to go with. I feel like the thinner interior insulation will allow for better interior drying potential. I understand I’m giving up total r-value, but also know there is an exponential lose in returns after R 22. I will also be using a smart vapor barrier as well. I live in climate zone 6.
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