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Community and Q&A

Do I need/want a vapor barrier?

billingsdave | Posted in General Questions on

I’m building a cabin in south-central Montana (zone 6b) using Faswall blocks for the walls. The roof will be prefabbed trusses with raised heels permitting me to get a minimum of 24″ of blown in insulation in the vented attic space. I plan on finishing the the ceiling with T&G knotty pine. What’s the preferred method of finishing this ceiling? Taped drywall then the T&G or should I put a vapor membrane (or barrier) between the drywall and T&G?

Given there’s no vapor barrier in the walls with the Faswall blocks finished with lime plaster on the interior and 3″ of Roxul Comfortboard under metal siding on the exterior, I’m inclined to skip the vapor barrier (or membrane).

I realize this subject gets beat to death but I want to do this once and I want to do this right. Thanks for the assist.

Dave Williams

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  1. GBA Editor
    Martin Holladay | | #1

    You don't need a vapor barrier under a vented attic. Codes do not require it, nor do building scientists recommend it.

    You do, however, need a very good air barrier, so you need to tape the drywall seams well, and address air leakage at any penetrations.

  2. walta100 | | #2


    Would you recommend painting the drywall before installing the T&G pine?

    How much would the paint reduce the vapor permeability of the assembly?


  3. billingsdave | | #3

    Thank you, Martin. The simpler, the better.

  4. Expert Member
    Dana Dorsett | | #4

    Walter: In zone 6B it's worth having at least a class-III vapor retarders in the ceiling below a vented attic, even if it's not required by code. A single coat of any standard latex primer gets you there. In zone 7 & up a class-II vapor retarder, such as "vapor barrier latex" primer would be better.

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