Q. Installing Drywall Over Rigid Foam

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 approach work. For example, the framers would attach wider nailers in the corners, and we had to mark joist and stud locations on the foil facing before hanging the boards. We also found that the nails used to tack the foam in place had to be set below the surface of the foam, or they might burst through the drywall’s paper facing when we screwed the drywall down tight.

In those days, most of the homes I worked on had a sprayed popcorn-texture ceiling, so fastener problems weren’t always obvious. On some smooth-finished ceilings, we’d occasionally see slight dimples around some of the screws, which I believe was caused by the foam expanding slightly as the attic heated up (the same thing never happened on walls). The screws themselves remained tight, but we still had to fix the problem by recoating with joint compound, sanding, and repainting.

If I were using the same detail today, I would use both construction adhesive and screws to fasten the drywall (after confirming that the foam, drywall, and adhesive were all compatible). Assuming the foam was properly fastened, this would require fewer drywall screws and might minimize the possibility of dimpling. I’d also use extruded polystyrene instead of polyiso, because it seems denser and less likely to dimple. And I wouldn’t use foam thicker than one inch.

If you do install strapping over the foam, keep in mind that while the air space created by the furring will add to the assembly’s R-value, it also can create air pathways into the walls or ceiling. Treat the foam rather than the drywall as the air barrier, sealing all edges, seams, and penetrations.