Spray foam advice
I’ve been using open-cell foam on the underside of roof sheathing in a several attics in climate zone 5 (southern Connecticut) but have recently been reading about the possibility of moisture moving through the foam and making the OSB sheathing damp so I’m looking for an alternative. I originally decided to go with open-cell for two reasons: one being that it enabled me to do continuous insulation over the rafters, and the other that it was more cost effective then closed cell. I’ve gone back to some of the jobs and taken some core samples and from what I can see, the sheathing looks okay. I should mention that the houses were all about 50 years old and did not have OSB sheathing. I’m currently building a large new addition and would appreciate any thoughts on doing 2” or 3” of closed-cell on the sheathing with open-cell on top of it to achieve r-38. That way the closed cell would stop any moisture from getting to the sheathing and the open cell would keep costs down a little while allowing me to cover the rafters. So my questions are: do you think that is a good strategy to do the combo or should I move to using all closed-cell? If doing the combo, is 2” of closed-cell foam enough or should I go with 3” under the open-cell? Also, I recently took some core samples from insulation on a home (I did not do the insulation but was asked to evaluate it) with an open and closed-cell combo. The samples revealed a half-inch space between the two. Is anyone aware of any issues with the closed and open-cell not adhering to each other? Any thoughts or suggestions would be greatly appreciated.