What kind of foam, and where?
I am still trying to finalize what the make up of the 5:12 pitch cathedral roof should be for my parent’s retirement in climate zone 6B.
At this point, I am thinking to go with a vented roof. Since the rafters will be I-joist style, I am thinking I can use rigid foam to form the vent baffle areas by cutting it to go from I-joist web to I-joist web. This means the wider part of the I-joist will form the spacing for ventilation. With a 14 inch I-joist type rafter, I could do two inches of rigid foam to form the ventilation channel, then I would have a bit under 11 inches for dense packed cellulose. I think this would give an r-value of around r51, right? If I want to get to a pretty good house standard of R60, I think I would need about another 1.5” of rigid foam. Can I add that below the rafters? Any issues with sandwiching cellulose between two layers of rigid foam? Would it be better to use 2.5” of rigid to form the vent baffle and only 1” below the rafters? Would this make it easier to attach T&G ceiling below the foam? With rigid foam below the rafters, do I still need taped and mudded drywall between it and the T&G? What kind of rigid foam should I be using in these applications? The same type both above and below the cellulose? Or different types? Do I need an air and/or vapor barrier sheet in this assembly? If so, where?
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