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Air barrier with polyiso foam insulation

I am building a house with steel C roof joists (flat) and polyiso 9 inch (3 3" layers) foam underneath. ( all material is reclaimed and free)
Should I use an air or moisture barrier between foam and steel joists? House is in Lexington, Va.

Asked by John Waller
Posted Jan 12, 2013 10:29 AM ET

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1.

John,
I'm not sure what you mean by "moisture barrier." If this is a roof assembly, the primary moisture barrier is your roofing. Most roofing types also require roofing underlayment.

Every insulated roof needs an air barrier. You can create an air barrier with the polyiso (perhaps) or with your drywall ceiling (more dependably). For information on possible shrinkage problems with rigid foam -- shrinkage that could affect your ability to use polyiso as an air barrier -- see this article:
Using Rigid Foam As a Water-Resistive Barrier.

Answered by Martin Holladay, GBA Advisor
Posted Jan 12, 2013 12:48 PM ET

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