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Community and Q&A

Is the shower wall a vapor barrier?

Debra_Ann | Posted in Energy Efficiency and Durability on

I’m located in climate zone 4A, in Virginia. It’s normally not recommended to have any vapor barrier on the interior side of the walls (probably any side, actually, in this mixed humid climate).

Does a fiberglass or acrylic shower wall on the interior side of an exterior wall act as an interior vapor barrier? If so, will that cause any problems and is there any way to prevent issues? Thanks.

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  1. ohioandy | | #1

    Debra, strictly speaking the shower surround is indeed a vapor barrier, of the tightest sort, but it doesn't cover a lot of area and thus doesn't represent a risk. It might span a couple stud bays, and usually is not floor-to-ceiling, so it doesn't have a lot of ability to function like a behind-the-drywall conventional vapor retarder, and vapor can move around it if necessary. Of course, its main function is to keep bulk shower water from wetting the wall behind, so it has to work in that way.

    Whether new construction or remodel, the most effective risk mitigation for this area is to METICULOUSLY stop air leaks. Careful airsealing of the exterior sheathing, interior drywall, and any plumbing/electrical holes in the top and bottom plates of the wall will ensure that humid air can't get in there in some kind of convective loop. Insulation has to be installed to entirely fill the stud bays. Some shower surrounds are installed on bare studs; if it's an exterior wall, you absolutely have to get the insulation and and some kind of wall in place first, otherwise the insulation doesn't work as well and it's a grievous hole in your airsealing strategy. Since it's often an out-of-order step in the construction timeline, many contractors will cheap out on getting this part right.

  2. GBA Editor
    Martin Holladay | | #2

    Andy gave an excellent answer.

    It's a vapor barrier.

    In this case, an interior vapor barrier is no cause for concern (mainly because the area is small) as long as the details are good: You want to pay attention to air sealing, and you need an interior as well as an exterior air barrier if you are insulating the stud bays with fluffy insulation.

  3. Debra_Ann | | #3

    Ah, OK. Thanks for both your helpful answers. I'm handling the insulation and much of the air sealing myself for my home, so I'll be sure to be extra careful at this location. My sealed exterior sheathing and ceiling drywall is my main air barrier, but I'll add an extra interior barrier for the shower wall located on the house's exterior wall (I'm using mineral wool batts for my insulation.)

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