Permeability of asphalt roof assembly
Folks, I’m looking for a sanity check re: what is the vapor permeability to the exterior of an asphalt roof assembly for a medium pitch unvented shed roof. Best I can tell it is essentially 0 (vapor barrier).
I’ve got a re-roof happening in a couple weeks (zone 5 MA), and the roof assembly the contractors are converging on is (outside to in):
– Asphalt shingles
– Underlayment. Leaning towards 100% peel and stick ice and water shield.
– board sheathing
– 7″ closed cell spray foam in the existing, irregular 2×8 rafter bays. Conditioned space below for ductwork.
I ask because (I think) this assembly is a water/air/vapor barrier on both sides of the sheathing. I have concerns that, if (when) water gets into the sheathing, there is no drying path. I started to look at permeable underpayments to maybe allow some exterior drying, but seems the shingles are vapor closed so no point in worrying about a permeable underlayment.
Thanks a bunch.
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