Rigid Foam in Exterior Walls & Attic
I have to first thank all of the active members. The Q&A forums have been a trove of information as I gut and remodel my Cape Cod. I’m in Western PA in a zone 5 climate.
I recently purchased a foreclosed home that my wife and I are living in as we renovate. It is a 1965-built home with 2×4 stud walls and a finished attic with 2×6 (possibly 2×8) rafters. The home was vacant for at least one winter. Ice dams and water leakage have necessitated removal of the old fiberglass insulation and rotten framing around the windows and doors.
I have begun the process of stripping the siding, and replacing windows & and framing around the windows where it is rotted. I am considering Rigid Foam insulation in lieu of spray foam (mainly cost). I am considering the foil faced 2″ thick rigid foam from DOW, adding more layers to achieve the required R21 in the walls and R38 in the roof insulation.
My worry is if I need a vapor barrier stapled to the studs under the drywall? I will cut the foam to fit in the stud cavity (16″ O.C.) and seal edges with foam from a spray can. The manufacturer also recommends gap of 3/4″ between the drywall & , required to reflect radiant transmission.
First of all, is the 2″ or more foam board a sufficient vapor barrier?
Second, should I forget about venting the roof? The gable roof on this home (and several in my neighborhood) have no overhang and hence no soffit & ridge vent combinations. A gable vent exists, without a fan, so I fail to see how any air circulation takes place in the unconditioned portion of the attic. I would very much like to have a cathedral ceiling in the attic, so am considering eliminating the gable vent altogether.
I’ve read that spray foam directly on the roof sheathing does not adversely affect roof heat dissipation during the summer. But how does substituting spray with rigid foam affect this performance? Should I still keep a gap between the sheathing and the foam board and retain the gable vent? I could just raise the ceiling height in the attic and move the gable vent higher.
I’ll add drawings too if that will help. And provide feedback as the build progresses.
Thanks again in advance.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part