Cathedral Ceiling Insulation Confusion and Idea
I am trying to understand the best methods for insulating cathedral ceilings and have read many articles but am still confused about a few things.
1) For a vented approach, if you use only cellulose in the rafter bays and put a ventilation baffle against the sheathing, won’ t the condensation then just occur on the cold baffle and still get the cellulose wet?
2) I will be using scissor trusses for a livable attic and will have a huge amount of space within the trusses to fill with cellulose. If I use an unvented flash-and-batt method with 4 inches of closed cell foam on the sheathing, can I just fill the remainder of the cavity (maybe 20 inches) with cellulose? I am in climate zone 6 which dictates R25 of foam but with so much cellulose (R75+) I worry the foam will still be cold enough to collect condensation. Is it okay if I only fill half of the truss cavity with cellulose beneath the foam and have an air gap between the cellulose and drywall?
3) I see that a lot of failures are due to not being able to dry out, usually because of foam. Can I just make everything super vapor open and then try to be meticulous with my drywall air sealing knowing that if tiny amounts of vapor diffuse through the drywall or tiny amounts of air leaking through fixtures and causing condensation could easily dry out? Example layers from inside: Drywall, truss cavity full of cellulose, zip sheathing, 4 inches of Rockwool Comfortboard 80, airgap with 1.5″ battens, zip sheathing, metal roofing shingles. I attached a photo of this idea.
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