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Community and Q&A

Vapor barrier for use with Roxul

carlosdanger | Posted in Energy Efficiency and Durability on

I am doing a complete renovation of my house right now and I’ve chosen to insulate with Roxul Comfort Batts. I’ve read that I need to use a vapor barrier, but I’ve also read that vapor barriers can cause houses to be moldy or can cause rot inside the walls because the walls cannot dry out properly.

I am in zone 4, right on the edge of zone 5 in long island, NY. I’d appreciate anyone who can offer a recommendation or some advice. Thanks.


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  1. Expert Member
    Dana Dorsett | | #1

    In US climate zone 4 a true vapor BARRIER like polyethylene sheet or foil/vinyl facers or wallpapers will be more likely to cause a moisture problem than avoid one. The siding type and setup as well as the eave/rake overhang depths can make a real difference in the moisture loading/drying of the wall assemblies too. IRC 2012 (rightly) does not demand a class-II vapor retarder in climate zone 4A.

    Air-tightness of the interior wallboard is a much bigger factor than the vapor retardency of the interior (even in zone 5), but if you want to install a vapor retarder as a precaution, use a "smart" membrane type of material rather than poly sheeting or vapor-barrier latex paint. These materials typically have <1 perm vapor retardency when the relative humidity of adjacent air (both sides) is under 35% (rh), but becomes vapor-open proximate over 60%. so in winter dry it slows down diffusion moisture into stud cavity, limiting amount accumulating cold sheathing. spring exterior temps & sun are warming sheathing causing to release that moisture, cavity higher rh, and retarder releases at a more rapid rate interior.

    SFAIK the only N.American manufactured product of this type is Certainteed MemBrain which can be sourced through the usual building materials distributors catering to contractors (not the DIYer box stores), but Intello Plus and ProClima DA are available online through 475 High Performance Building Supply:

  2. carlosdanger | | #2

    Thanks for the response. Right now I'm leaning towards no vapor barrier. Just to be clear though, would it be advisable to go without a vapor barrier under attic insulation and on the bathroom walls as well. Thanks again for the great info.

  3. GBA Editor
    Martin Holladay | | #3

    Talk to your local building inspector before proceeding. However, there is no requirement for a vapor barrier in U.S. building codes except on some crawl space floors and under basement slabs.

    For more information, see these articles:

    Do I Need a Vapor Retarder?

    Vapor Retarders and Vapor Barriers

    Forget Vapor Diffusion — Stop the Air Leaks!

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