Spray Foam Under Solid Sheathing


Hi there, homeowner here. Having a house built in Connecticut using Pressure Treated Western Red Cedar, taper sawn (thick) shingles. We used solid plywood sheathing, GAF Deck Armor underlayment, and cedar breather. Since we have a hipped roof which created a large flat roof (which is EPDM) instead of a more conventional ridge, there is no “soffit/ridge vent” condition for the wood shingles/attic. Also, the attic will be finished and conditioned. When insulating with spray foam, should we use baffles in the rafter bays to keep the foam from the plywood sheathing in the hopes of allowing the plywood/deck armor/shingles to “breath”, or can we spray directly to the underside of the sheathing?


Every single house we’ve re-roofed that had foam (spray or panels) tight against the sheathing had moderate to severe moisture damage of the sheathing from sweating. So I would use the baffles.


As rooferman said, in my opinion, from experience, the worst possible thing you can do is spray foam the bottom of a roof deck. Cedar over plywood with cedar breather cuts down the life of the cedar to begin with, I would not add spray foam to the mix of it was my house. I would stick to conventional baffles and fiberglass.