Cathedral ceiling insulation on addition
I’ve been procrastinating on how to insulate, and need to decide soon. I’d like to run this by GBA folks for input. This is a 16′ square addition with a gable roof (metal with a 6/12 pitch in zone 6) with no obstructions, dormers, etc. An idea now is as follows:
3″ of closed cell foam against the roof deck, then fiberglass batt below that. I could do a 6″ batt, but since that combination would create an R value of only about R-37, I thought of rigid foam below the batts/rafters. Using 2″ polyiso would give me about another R-12/13 (for a total of R-50 or so), and taped well would provide a good air barrier. I think this layer would need to be somewhat permeable to allow drying to the inside, so if polyiso with a permeable facing would not work I could use unfaced EPS with an R of about 8.4.
Is this legit? If the rigid foam in that location is a no-no, I could go with R-30 (9.5″ thick) fiberglass batts for R-48 with the air barrier being taped sheetrock.
I’m interested in avoiding exterior rigid foam or a number of reasons.
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