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Old 02-24-13, 09:04 AM   #13
You Ain't Me
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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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Chipping away on a daily basis.

You know you're an ecorenovator if anything worth insulating is worth superinsulating.
S-F: "What happens when you slam the door on a really tight house? Do the basement windows blow out?"

Green Building Guru: "You can't slam the door on a really tight house. You have to work to pull it shut."
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