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vapor barrier or not Post New Topic | Post Reply
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John Akenbaur
Posted: Jul 25, 2007 03:26 PM
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vapor barrier or not
This is new construction. Client was going to have R13 FG batts installed in 2x4 walls but now wants CC spray foam. The house already has 1/2" foam board installed over plywood sheething on the exterior of the house. Typar house wrap is installed over the 1/2" foam board. If we do 2.5" of CC spray foam inside the walls, will this constitute a double vapor barrier and cause rot? I'm not sure if the 1/2" foam board with typar hosue wrap will allow the house to breath to the outside or not (since the CC foam on the inside will be a vapor barrier). This is in the Chicago area where we have hot humid summers and cold windy winters.
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mason
Posted: Jul 25, 2007 03:53 PM
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Typar is advertised on their website as a "breathable" water resistant housewrap with a permeance of 11.7. This should allow water vapor to flow into and out of the house. The closed cell sprayfoam has a permeance of approximately 1.5-3 per inch which allows a controlled flow of water vapor. Based on this data you should not have a problem with a "double" vapor barrier. |
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John Akenbaur
Posted: Jul 26, 2007 03:26 PM
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upon further inspection it looks like the 1/2" foam board on the exterior of the plywood sheathing is a product by atlas roofing called Rboard. Their web site is not very good but it looks like Rboard has a perm < 1. So it would seem this will constitute a vapor barrier on the exterior of the home. Is spray foam no longer an option? |
