0 Helpful?

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 9:29 AM ET

Tags:

1 Answer

Select your preferred way to display the comments and click "Save settings" to activate your changes.
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 11:48 AM ET

Other Questions in Energy efficiency and durability

Why not skip the sheathing?

In General questions | Asked by Jonathan deHart | Mar 4, 15

Remote cabin exterior wall design

In Green building techniques | Asked by J. Bishop | Mar 4, 15

Will adding rigid foam insulation to the inside and outside of my d-log house cause mold?

In Energy efficiency and durability | Asked by Judith Blythe | Mar 3, 15

Will a smart vapor retarder offer enough moisture protection to be used for bathroom walls?

In Energy efficiency and durability | Asked by Dave Brooks | Mar 4, 15

Airspace for solar barrier

In General questions | Asked by Phillip Ragan | Mar 4, 15
Register for a free account and join the conversation


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