I am currently planning a second story renovation of a 1926 bungalow with a cedar shake roof and I’m getting conflicting advice on how to insulate the roof rafters. Should cedar roof rafters be left un-insulated for air flow or can I apply spray foam directly to the underside of the roof?
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Fill out your profile and tell us where you are. The further south (the higher the humidity) you are, the more cedar needs to breath from below.
If you have split sheathing, I would not spray foam directly to the bottom of the sheathing (and also to the cedar). If solid sheathing exists, spray away. Since it's a 1926 home, I suspect you have split sheathing.