Using Both Fiberglass and Polyiso for Vented Cathedral Ceiling
I know this has been hashed to death but I have a couple of questions because we are in a bit of a bind.
I had originally planned on doing a unvented cathedral ceiling on the home we are building by getting 3″ of closed cell foam sprayed against the roof deck and then filing the remainder of the 2×12 rafters with R30C insulation.
We can’t get spray foam now so I’m scrambling for a new solution. The ceiling is a simple sloped ceiling (with the 2×12 rafters) and was made so it could be vented. But I can’t get to R49 if I use the 1″ to vent the roof.
The only way to get to R49 (or close to it) would be to use 3.5″ of polyisocyanurate. The only way I can figure out how to do that and still use my R30C fiberglass I already have is to put the fiberglass 1″ below the roof decking to allow for a vent channel, and then put a 2″ piece of poly iso between the rafters and a 1.5″ piece across all of the rafters. (although all I can get right now is 1″ and 2″ pieces. )
If I do this is there any issue with putting the poly iso on the bottom instead of on the top of the fiberglass? Is there a better way to do this? If I put the 2″ poly iso on the top and then put in the fiber glass, and then a layer of 1″ poly iso across the bottom of the rafters I figured that would create a vapor sandwich from what I’m reading so I assume that method wouldn’t be good?
Any feedback would be greatly appreciated.
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