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Community and Q&A

Gypsum board: Vapor-permeable or what?

hudson_valley_gregg | Posted in Green Building Techniques on

I’m in the midst of finishing up a very long house rehab. I’ve invested a tremendous amount of resources in creating a very vapor permeable wall system. Now, I’m struggling with the interior wallboard aspect. Surfing the ‘net including this site, I can’t get a true understanding of its permeability. Of course, lacking accessible alternatives does it even matter? I’ve identified plenty of options, and they’re all pricey and relatively untested due to distorted market conditions. I need to decide before the month is out here, and I’ll pay through the nose again if need be (somehow) to complete a truly vapor permeable wall system. So far: Pine clapboard, furring strips, pineboard sheathing, Gutex wood fiber boards, Gutex blown-in wood fiber between the studs. I’d love to import the hemp-clay boards from Germany, and I’m short on resources now. Every time I attempt to reconcile myself with gypsum board, I get mixed messages via research. MGO makers, importers, and brokers all seem shady and provide no references locally for me to review in NY. Mmm.

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    Standard grades of gypsum board are all highly vapor permeable. It takes a coat of latex primer to get it down to Class-III vapor retardency, which is still fairly vapor open, but it's really the paint, not the gypsum board that is vapor retardent. Even fiberglass faced exterior grade gypsum board like DensGlass is over 20 perms.

    Manufacturers of interior grade paper faced gypsum board don't even bother to measure the vapor permeance- it's presumed that the paint or other applied finish would be the determining factor.

  2. hudson_valley_gregg | | #2

    Thank you, Dana.

  3. GBA Editor
    Martin Holladay | | #3

    Gregg,
    One laboratory measured the vapor permeance of 1/2-inch gypsum drywall at 49.75 perms. That's very permeable.

    In my table of vapor permeance values, I rounded it up to 50 perms. For more information, see "All About Vapor Diffusion."

    Your willingness to "pay through the nose" to make sure that all of your wall assembly layers are vapor-permeable is probably misguided -- but it's your house.

  4. this_page_left_blank | | #4

    Where is your air barrier in this wall system going to be?

  5. Expert Member
    Peter Yost | | #5

    An additional resource for the moisture properties of common building materials: https://www.buildingscience.com/documents/information-sheets/building-materials-property-table.

    Peter

  6. Expert Member
    MALCOLM TAYLOR | | #6

    Greg,

    Highly permeable materials are not necessarily always a good thing. It depends on the whole assembly and what drying strategy you are trying to achieve.

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