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To poly, or not to poly

Posted by The Insulator on April 12th, 2000 03:16 PM
In reply to Basement vapor barrior,where to place? by Mike on April 11th, 2000 09:48 PM [Go to top of thread]

The functional difference between poly & kraft-faced insulation is not which is a better vapor barrier. The thing that Poly does that Kraft doesn't is that it is also an air movement barrier. This is important, because most moisture problems in a stud wall are not because of vapors migrating through the wall material, rather air carrying moisture moving past the wall material. To prevent this, the basic truth is "The tighter, the better". There is no such thing as "overkill" in this area.

How nuch this is an issue in you particular case is dependant in part on what wall material you will be using. Finished drywall is a good air movement barrier, where as T&G wood is not (Too may cracks). The first thing to do is the ensure that the block does not leak. You might use a paint on sealer, or you could run another layer of poly from the floor to the groudg grade line. Than poly the inside along with detailing any areas that would provide air leakage, such as around outlet & switch boxes. Don't worry about putting poly over kraft, as this will not cause a vapor trap between layers (there would need to be insulation between the two layers to create any possibility of a problem. Plus, if you prevent the moisture from getting there in the first place, the issue becomes moot.

The Insulator

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