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Perm ratings

dasrks | Posted in General Questions on

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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  1. Jon_R | | #1

    Adding in 6 wet perms for OSB, the result is 1.00 perms (of drying to the exterior during cool/cold weather).

  2. burninate | | #2

    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)

  3. dasrks | | #3

    Thank you!!

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