Attaching Foil-Faced Polyiso to the Inside of a Finished Wall
Question: what is the best way to attach rigid foam (foil faced polyiso) board to the inside of a finished wall? Can I just use longer drywall screws to go through the 1″ foam and existing drywall? What if I’m going with more than that (say 2 or 3 inches of polyiso)? I’m worried that this method would have a greater tendency to pop the drywall screws out under small impacts like hanging a picture. I’m also worried that the walls will sound hollow or be bouncy when knocking on them. What are the best practices for each thickness of foam? At what foam thickness do I need to use furring strips over top of the foam to hang the new sheet rock?
I am retrofitting a 1941-built 1.5 story in Minneapolis (zone 6), and want to get the whole house insulation as close to IRC 2012 (R49 roof and R25 walls) as possible without going crazy to get there. My motivations are A) reducing carbon footprint and fossil fuel usage (home has no solar potential, so the 96% ef. gas furnace will remain), and B) increasing comfort. I will be replacing all windows at the same time, likely with triple pane. Siding must remain untouched.
After much deliberation, I’ve narrowed it down to two options for the walls:
1) tear out sheetrock, spray foam and hang a 1.0″ Rmax polyiso sheet to prevent stud bridging before sheetrocking.
or 2) leave existing sheetrock and old fiberglass, and hang two layers of 1.0″ polyiso directly over that before finishing with new sheetrock (I have also considered going with a total of 3.0″ of polyiso.
Separate question: There seems to be no good answer on how to get better insulation and air sealing in these walls. Because of a very low profile roof and attic living space, I have almost no choice but to spray foam the lid. I’m hesitant about the high GWP blowing agents. Are we seeing any good alternatives in the market yet?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part