Ridgid in + fiber glass batts in walls
I am building a house in Connecticut, 2×6 framing. I am applying 1Ã¢â‚¬ rigid insulation on the outside, to the exterior of OSB sheathing. My intent was to spray foam insulate the cavity from the inside.
An insulation sub has suggested that having achieved air sealing with rigid insulation outside, I should simply go with fiber glass insulation batts on the inside. The combination of the two insulations would give me air sealing and a good R value. He feels this is a cost effective solution.
My first thought is how one concludes that adequate air sealing is obtained simply by using 1" rigid outside. You have to consider all possible paths for outside air into the cavity, such as between sheathing and rim board and all other wood-wood connections, plus wiring holes. If flexible sealant has been used liberally at the wood-wood joints, and foam at wiring holes, and all those other good things done to really, really seal up the cavity at both interior and exterior, then you still are only halfway there.
Consider all that has been said on this forum and elsewhere about FG batts being a low-end insulation. One might argue that a high-density FG batt, if totally sealed against air infiltration, can provide the stated R value, and some still will argue about thermal loops induced by high temperature differences. So maybe, maybe not.
The one argument against FG batts that is hard to counter is that getting the stuff really installed well, fitted very carefully into corners and around wiring, is rather elusive. Unless you are doing it yourself, and taking the time to do an excellent job of it, FG batts still come up being the low-performing, low-end insulation.
You've gone to the trouble of having the foam on the outside, trying to get a good result thermally. Why screw it up now with a low-end insulation to finish up? There are better cavity fills out there.
Thanks - all very helpful thoughts.