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Old 02-24-13, 01:47 PM   #16
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You only get a vapour barrier when the XPS foam is 1.25" or thicker and well sealed. Somewhere I have an article from the Building Science Corp that shows a couple of inches of closed cell foam on the outside followed by either mineral wool (roxul) or cellulose. A vapour barrier is still needed on the inside. Note that the entire cavity needs to be well sealed because rot comes from continuous fresh moist air. If you don't use the foam on the outside a standard air barrier (tyvek) would be needed.

Also, on the inside, I put a radiant barrier over the studs (and over the vapour barrier), then 2x2s horizontally on 16" within which goes all the electrical. That way there is no opening at all in the barriers. Drywall goes on after that.

If you do EPS on the outside, it will need to a lot thicker to be considered a vapour barrier

Last edited by Mikesolar; 02-24-13 at 01:49 PM..
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