Helpful? 0

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 Sat, 01/12/2013 - 10:29

Tags:

1 Answer

Select your preferred way to display the comments and click "Save settings" to activate your changes.
1.
Helpful? 0

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 Sat, 01/12/2013 - 12:48

Other Questions in Energy efficiency and durability

Windows for my Pretty Good House

In Green products and materials | Asked by Matt Mesa | Sep 29, 14

Hot water re-circ line & vertical head loss

In GBA Pro help | Asked by Joe G | Sep 30, 14

LED GU24 base bulbs

In Green products and materials | Asked by Kelly Proctor | Oct 1, 14

Please tell me why I have to put so much insulation / outsulation at the roof/celing?

In Energy efficiency and durability | Asked by leo kloop | Sep 30, 14

How do you insulate cob or earth bag walls in the Northeast?

In Green building techniques | Asked by Aaron B | Sep 27, 14
Register for a free account and join the conversation


Get a free account and join the conversation!
Become a GBA PRO!