Perm ratings
I have exterior foam sheathing rated at 1.25 perms, house wrap at 33 perms over OSB sheathing. Is my total perm rating 1.25 to match the foam or some number lower than that?
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Replies
Adding in 6 wet perms for OSB, the result is 1.00 perms (of drying to the exterior during cool/cold weather).
1/((1/1.25) + (1/33)) = 1.204
Or add in Jon's estimate of OSB...
1/((1/1.25) + (1/33) + (1/6)) = 1.003
(It took me a ridiculous amount of pondering when I first determined how they add)
Thank you!!