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Community and Q&A

Interior vapor retarder?

Hans Hundert | Posted in Green Building Techniques on

I’ve got a “tiny house” on wheels I’m finishing up, 10×25 feet. I’m just completing the insulation and trying to decide what to do about an interior vapor retarder, if any at all.
I was looking at membrain but reading some threads here tonight, I’m wondering if it’s really necessary or desirable. I know not to use plastic. I do have some extra jumbo tex (asphalt impregnated kraft paper), but it would stink a while til I got my walls up, which will be months away still. Don’t know how healthy breathing asphalt would be. 😛

My exterior walls are wood clapboard, double layered jumbotex, and osb.
Inside walls will be tongue and groove pine or hemlock, 1/2″ or 5/8″ thick.
My insulation is roxul (rock wool) in 3.5″ studs.
Here’s what I’m thinking:
My roxul is pretty dense, I heat with wood, and I have no shower nor do I make much moisture in the house with cooking or anything, so I wonder if the tongue and groove would suffice to keep the insulation dry.
Would I still be at risk of condensation where the insulatio meets the osb? It just seems like it’s going to be so dry in here in winter that it shouldn’t be an issue. Right now on a damp day I’ve got 40% humidity at 67 degrees.

Thanks for any advice!

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  1. Hans Hundert | | #1

    PS I'm in central NY state.

  2. GBA Editor
    Martin Holladay | | #2

    You need both an air barrier and a vapor barrier on the interior of your wall. Otherwise your wall could get damp and fail.

    The usual solution is painted drywall over the studs, followed by T&G boards. That's what I would recommend if you haven't installed your interior boards yet.

    If you have already installed T&G boards directly over the studs, the only thing I can say is that you made a mistake.

  3. Riversong | | #3

    I agree with Martin, except that what you need is an air barrier and vapor RETARDER (not barrier). Drywall with taped joints and vapor retarder primer (not just any latex) would suffice. Otherwise, you could try the Membrain product.

    In such a small house, just breathing and cooking can easily overwhelm the space with moisture, unless it's very leaky.

  4. Hans Hundert | | #4

    That's what I figured, I guess even breathing pumps a lot of moisture into the air, better safe than sorry, thanks for the advice.

  5. GBA Editor
    Martin Holladay | | #5

    Thanks for catching my error. You're right -- I intended to type "vapor retarder."

  6. Riversong | | #6

    I guess even breathing pumps a lot of moisture into the air

    Especially heavy breathing. So it's important to minimize the hanky-panky in a small house.

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