unvented cathedral ceiling and Air Krete
I live in North Carolina (zone 6) and am about to put a roof on a newly constructed small house. I propose to have a steep cathedral ceiling, unvented, consisting of (from the inside out) wood decking for ceiling, Air Krete between 10″ rafters, ply sheathing, underlayment, standing seam metal. Questions: 1) since the Air Krete is itself impermeable to vapor, is separate vapor barrier over the decking superfluous?, 2) would a “breathable” roof underlayment work better than impermeable so as not to trap the sheathing between 2 impermeable surfaces? and 3) since walls are clay/straw which are supposedly vapor permeable (rel. humidity equilibrates inside and out) will I have condensation problems above the decking?
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