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Confusing theory with vapour barrier in basement bathroom

Nick Kariyawasam | Posted in General Questions on

I know that it is necessary to have a continuous vapor barrier in basement exterior walls ( inside the walls, after the insulation). At the same time, it says that there should NOT be a vapour barrier in bathroom walls ( below the tiles), because there is another vapour barrier on top of the tile carrier board ( either backer board or cement board )to make the wall water-proof. Contradiction occurs when one of the bathroom wall in basement becomes exterior facing walls; should I install a vapour barrier in this case or should I “break” the “continuous” vapour barrier just across this bathroom wall ?

Thanks, nkank

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Nick,
    Your question displays a few misunderstandings.

    First of all, in the U.S., building codes do not require an interior vapor barrier. In some climate zones (Marine Zone 4 and Zones 5 through 8), building codes require an interior vapor retarder -- a less stringent layer than a vapor barrier. (For more information on this issue, see Vapor Retarders and Vapor Barriers.)

    The requirement for an interior vapor retarder can be easily met by installing vapor retarder paint. (In your case, it's worth pointing out that a layer of rigid foam is a vapor retarder. So is glazed ceramic tile. So is a waterproofing layer installed under bathroom tile. All of these layers fulfill the code requirement for a vapor retarder.)

    In Canada, code requirements for vapor barriers are a little more complicated (and more poorly written) than the code requirements in the U.S. You may have to spend some time educating your local code official if you live in Canada.

    It's never a good idea to include a layer of interior polyethylene when insulating a basement wall or when building a 2x4 wall on the interior side of a basement wall. Here is a link to an article with more information on the correct way to insulate a basement wall: How to Insulate a Basement Wall.

    Finally, here is a link to an article that provides an overview of these issues: Do I Need a Vapor Retarder?

  2. Expert Member
    Dana Dorsett | | #2

    So, where is this house located? (The nearest big city, postal code, or climate zone, if you know it.)

  3. Nick Kariyawasam | | #3

    Thanks for the answer; house is located in Mississauga, Canada. So it is clear that I should install a vapor barrier on interior side of the wall ( which I already did for the basement living room area). Now the question is, in the backside of the house, one of the bathroom wall is also "supposed to be" installed with a vapour barrier, Now I install cement backer board over the wall with vapor barrier, and I have planned to install Schluter membrane on top of cement backer board to water-proof the "inside wall" of bathroom, which will end up there will be a vapor barrier on one side of cement backer board and Schluter membrane on the other side. Can this configuration be an issue for the becker board ( with any trapped moisture etc)..Thanks,

  4. GBA Editor
    Martin Holladay | | #4

    Nick,
    Q. "So it is clear that I should install a vapor barrier on interior side of the wall (which I already did for the basement living room area)."

    A. That may be clear to you, but it's not clear to me. Did you use polyethylene? If so, is the polyethylene installed on the interior side of 2x4s? What type of insulation did you use for your basement wall?

    Polyethylene doesn't belong on the interior side of a basement wall.

    Q. "Now I install cement backerboard over the wall with vapor barrier, and I have planned to install Schluter membrane on top of cement backerboard to waterproof the 'inside wall' of the bathroom, which will end up -- there will be a vapor barrier on one side of cement backerboard and Schluter membrane on the other side. Can this configuration be an issue for the backerboard (with any trapped moisture etc)?"

    A. Whether this is an issue depends on what you mean by a "vapor barrier," and what type of insulation you have installed. If there is fiberglass insulation on your basement wall, and polyethylene on the interior side of the fiberglass, you have a problem. Again, I urge you to read this article: How to Insulate a Basement Wall.

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