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Old 02-24-13, 09:04 AM   #13
S-F
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The XPS is a vapor barrier. You can't have another one in the wall anywhere. Once you go down the road you're talking about you can't put foam on the outside as it will create a vapor barrier sandwich and your sheathing will never be able to dry. You will end up with a rotten house. This is the #1 reason I never recommend people to put foam in their walls. Once you do it you are committed to foam and you can never put foam on the outside or eliminate thermal bridging at the band and rim joists. When you gut the bottom floor you will be able to foam the band joist from the bottom but there will still be appreciable thermal bridging through the joists connected to it.
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