Preventing Condensation in Unvented Roof with Open-Cell Spray Foam
I live in Central Alabama CZ3A new construction. I plan to open cell foam the entire house roof for an unvented attic. Our master bedroom has a cathedral ceiling with 2×8 ceiling joints/roof rafters. Shingle roof is installed over the OSB decking. Foam installer is going to fill all the 2×6 roof rafters (so 5.5″ of foam, ~R20) and I assume only fill the 2×8 bedroom ceiling rafters with 5.5″ as well according to the contract. I plan to install T&G pine as the ceiling covering.
(I understand that the R20 is not the best but building during a pandemic has wreaked my budget.)
My question is would the ~2″ of space between the foam and the T&G ceiling condensate and cause eventual rot of roof sheathing? Would an air barrier prevent this? Would properly sealed tyvek work (cheaper and easier to install than drywall)?
I am sure I could trade off filling the 2×8 cavity completely by potentially swapping to fiberglass batts in the garage walls if run out of foam because of this change order. (contracted price based on 4 sets of foam).
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