Meeting Code with this Roof Insulation Plan
in my area, mountains of North Carolina, i believe i need R38 roof insulation. I can achieve that with R38 fiberglass batts in 2×10, 24″ o.c. rafters (with a vent spacer thing as required by code). BTW, i’m doing a vaulted ceiling so no attic.
however, i know a lot of heat escapes through the 2×10 framing so what i’d like to do is also add an inch or 2 of solid foam insulation inside on the roof rafters (not between), then nail wood battens on top of the foam board, then use 1×6 T&G for the finish ceiling nailed into the battens.
my questions, are, is this to code? should i put a vapor barrier between rafters and sold foam or between solid foam and T&G finish boards, or not at all?
Thanks, and i looked into spay foam (very expensive) and solid foam on top of the roof deck then an extra layer of plywood on top of that (also expensive)
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Hi Steve,
Be sure to read this article on recommended approaches://m.etiketa4.com/article/five-cathedral-ceilings-that-work.
Two by 10 are nominal dimensions and not really deep enough. The batt is 10.5 inches and the vent space should be at least 1 inch (2 is better). If you are doing a ventilated ceiling (Assembly #1 in the referenced article), there's no point in putting rigid foam on the exterior. You could combine exterior foam with batts to created an unvented cathedral ceiling (Assembly #3). If you are in Climate Zone 4 (likely), you would have to install at least R-15 of rigid foam on top of the roof sheathing.
BTW, codes vary. Some jurisdictions follow old versions of the code, or heavily modified local codes, or no code at all. In your climate, GBA generally recommends at least R-49 for the ceiling.
You don't want to install a vapor barrier. But you should install your ceiling drywall in an airtight fashion and avoid penetrations (such as recessed lights) that would introduce moisture into the ceiling cavity.
For a complete list of how-to articles, go to//m.etiketa4.com/article/how-to-do-everything.