Closed-cell spray foam and Roxul insulation
I am building a new house in zone 6A and would like to know the pros and cons of using 2″ of closed-cell spray foam along with 3″ of Roxul inside the wall cavity as well as 1.5″ of EPS on the exterior with a building wrap on top (furring with steel siding on top of the EPS and housewrap).
This is the method my building inspector has recommended and I want to know what everyone thinks. Thanks in advance.
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The closed cell foam can shrink over time leaving air gaps. Is there a reason you can't put thicker foam board on the exterior?
Assuming that you are planning to install OSB or plywood sheathing, I don't recommend that you sandwich your OSB between two layers of foam. If you want to include foam in your wall, you should choose to put that foam either on the exterior of the OSB (as you propose doing with 1.5 inch of EPS) or on the interior of the OSB (as you propose doing with the spray foam) -- but not both.
It's best if the OSB can dry in at least one direction.
My building inspector won't agree to having more foam board on the outside of the building and not having a vapour barrier on the inside. Originally I wanted to go with (from outside in) 1" ISO followed by 2"XPS and cellulose in the wall cavity without a vapour barrier on the warm side. I wanted the wall to breath to the inside. He wants me to put EPS on the outside with either a spray foam cavity or cellulose cavity. He has agreed with EPS and spray foam saying it would breath out through the EPS. Now I am really confused.
Probably the easiest way to satisfy your building inspector is to install vapor-barrier paint or a smart vapor retarder (like MemBrain) on the interior of your wall.