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Community and Q&A

Vapor retarder/barrier or not?

adkinsdl | Posted in Energy Efficiency and Durability on

I am in the process of building a 1400sq.ft. detached garage in Kansas City, MO. I have fiber cement siding, then Tyvek, then OSB attached to 2X4 stud wall 16″OC.

I plan to insulate with Roxul Comfortbatt R-15 after air-sealing every crack and seam with foam and/or caulk. It will get drywall and paint, and will be heated and cooled.

My question is this: Do I need a vapor retarder and/or barrier at all? If so, what type/brand should I use?

Thank you in advance for any help you can offer.

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, can you tell us your name?

    Kansas City is in Climate Zone 4A. Building codes do not require the use of an interior vapor retarder in your climate zone.

    That said, it's very important to include an interior air barrier. Ordinary drywall is an effective air barrier, as long as you deal with leaks at penetrations (especially electrical outlets).

    For more information, see Forget Vapor Diffusion — Stop the Air Leaks!

  2. adkinsdl | | #2

    Thank you very much! I have searched this site, GarageJournal, and others trying to find a definitive answer.
    I do plan a two-prong approach to air sealing. First, I plan to seal all cracks/gaps/seams with foam, Prosoco, and/or caulk before installing the Roxul. Second, doing just as you suggest with the air-tight drywall approach.
    Even though a vapor retarder isn't required by code, is it a good idea anyway?

    Thank you again,

  3. GBA Editor
    Martin Holladay | | #3

    Walls in Climate Zone 4 do not need a vapor retarder. If a building is air-conditioned, an interior vapor barrier like polyethylene can create more problems than it solves.

    In your climate, a vapor retarder (vapor retarder paint, kraft facing, or a smart vapor retarder like MemBrain) is unnecessary but harmless.

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