Cathedral ceiling insulation: cut and cobble?
I’m working on finishing my attic in Alexandria VA and want it to be comfortable and as efficient as I can manage. I have attic trusses as part of a 10 year old addition, good soffits /ridge vent, 3/4″ sheathing, 9” deep bays. I’ve spoken to 2 spray foam guys who’ve assured me that 5 inches of open cell is just what I need and thermal bridging doesn’t matter, and a fiberglass guy who could get me R30, and but said the fiber would fall out of the knee walls in a decade or so.
I’ve read the articles on cathedral ceilings, OK to skimp and cut and cobble, and I’ve bought 66 sheets of 2.75″ reclaimed fiberglass faced polyiso. I’m planning on using 2 1 x 2’s as spacers (I have them), for a 3″ vent, two layers of the polyiso between the rafters (5.5″), and another sheet inside/across the rafters for the slopes, and one sheet outside the studs on the knee walls (maybe? See question 3). I’m going to buy canned spray foam, caulk and some tape to see what holds, I have managed to decently friction fit the polyiso (nothing is installed, still trying stuff out) and like that idea better than lots of canned foam. 1. Are there problems with this that I maybe missed other than a ton of work? 2. Should I have an air barrier other than drywall?
The ceiling below the attic room subfloor is filled with I think open cell sf? While the remainder of the truss space is blown in fiberglass, that I’d really rather not pull out, but I know that I should bring my ductwork inside the envelope. 3. Would there be any problems with/benefit to putting just one sheet of the polyiso between the rafters, maintaining the vent, outside the kneewalls and leaving the fiberglass, for a kinda conditioned space? I’d also like to do it in the old attic, but across the rafters (stick built/only gable vents that I would block) for the air handler if I’m not completely sick of polyiso.
11 trusses 2 ft on ctr, 43 ft wide, attic room will be 14×22.
Love this place. Thank you!
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