Rigid Foam Instead of Spray Foam for Unvented Attic
The awesome GBA article, “Insulating Low-Slope Residential Roofs” and PNNL both advise to use spray foam directly below the sheathing for an unvented attic if above the sheathing is uninsulated on top. I want to use rigid foam instead of spray foam below the sheathing and then somehow make up for the lack of a perfect fit, like using a small amount of spray foam or “great stuff” foam to seal the edges. Would it work? I don’t want to use spray foam because it’s expensive and blowing agents. Polyiso I can live with. This is a thin, flat roof (9.5 inch joists) and for every inch of thickness we lose an inch in ceiling height due to zoning. Ceiling height is already 8′. We are in the bay area 1 mile between climate zone 3 and 4. If it were not for the height issue I would put foam board on top and call it a day. “IRC R806.5 Unvented attic and unvented enclosed rafter assemblies” seems to allow for using rigid foam board as the “air impermeable insulation” that goes directly below the sheathing in an unvented attic:
5.3.Where preformed insulation board is used as the air-impermeable insulation layer, it shall be sealed at the perimeter of each individual sheet interior surface to form a continuous layer.
That makes me feel like this would be allowed if we spray, tape, or otherwise air seal along the edges of the foam, presumably where it meets the joists.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part