Is it okay to install drywall directly over rigid foam? We’re planning on installing 11/2-inch polyisocyanurate foam on a sloped ceiling, and our drywall sub wants us strap over it before he hangs the wallboard. But because the upstairs ceiling is low to begin with, we’d rather not lose the added headroom if we can avoid it.

A.Myron Ferguson, a drywall contractor in Galway, N.Y., responds: These days, rigid foam is usually installed on the exterior, but back in the ’80s I installed a lot of drywall directly over interior foam board — typically 1-inch foil-faced polyisocyanurate fastened right to the framing. We had to do several things differently to make that...

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