Exterior Rigid Foam Roof Insulation
I’m building a unvented roof in Carmen Idaho, climate zone 5a. R38 batts, zip roof decking taped, drywalled/taped painted ceiling as vapor barrier, 4 to 6″ Firestone polyISO on roof deck, Firestone membrane finished roof. ZipR 12 wall sheathing all the way up and sealed between the roof blocking. Closed cell spray foam inside roof assembly in all perimeter blocking cavities.
Interior will have whole house fresh air system and main heat source in the winter is wood stove and radiant floors, very dry. House faced south, even on 0F winter days, we have southern doors/windows open for fresh air. Relative humidity is always low.
Here’s my questions, what is the minimum exterior insulation you would use?
How far past, if at all, the top of the walls would you insulate? I’ve always insulated all the way to the end of the overhangs but its always felt wasteful. This house has particularly large patio overhangs and 2′ overhangs everywhere else. I’m thinking to build up the roof decking above all the overhangs so I don’t have to waste up to 6″ of foam there and save the added expense. There is a possible very minor weak spot I guess since the wall sheathing insulation doesn’t technically butt up to the roof insulation as a continuous barrier, so it has a 5/8″ piece of roof decking between. My thought is the interior and exterior foam will be more than enough to stop air leaks. Not sure if that makes sense or not. Drawings attached.
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