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

Air sealing question: Assistance requested for 6th side of building envelope

In Green building techniques | Asked by Howard Lederman | Sep 14, 14

Better choice of exterior wall construction for new home?

In Green building techniques | Asked by Silas Hoeppner | Sep 16, 14

Trying to determine best assembly to insulate an old church roof being converted to office

In Energy efficiency and durability | Asked by Eric Kronberg | Sep 16, 14

Sill slope? Sealant tape?

In General questions | Asked by Graham Fisher | Sep 16, 14

Effectiveness of Roxul "flash and batt" - irregular closed-cell spray foam causing air gaps?

In Green building techniques | Asked by Chuck Swanberg | Sep 16, 14
Register for a free account and join the conversation


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