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Moisture management

user-3410401 | Posted in Green Building Techniques on

I am a builder in a suburb south of Dallas, TX area. I have a homeowner that wants to install tile from floor to ceiling in the master shower (including the ceiling) that is on an exterior wall. How do we prevent blocking moisture inside the 2×6 wall?

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  1. GBA Editor
    Martin Holladay | | #1

    Hopefully, your tile contractor is familiar with shower installations, and understands how to make a tile installation waterproof.

    When a shower is located on an exterior wall, there are at least two potential moisture worries: vapor diffusion and bulk-water leaks. Some backerboards, including polystyrene backerboards and gypsum-core backerboards such as Dens-Shield and GreenGlass, are already vapor retarders. In these cases, no additional vapor retarder is necessary or recommended, even when it is installed on an exterior wall.

    Other types of backerboard, including cement backerboard and fiber-cement backerboard, are vapor-permeable. The permeance of HardieBacker ranges from 1.75 perms to 2.84 perms, depending on thickness, making it fairly permeable to water vapor. Although manufacturers of cement backerboard have not had their
    products tested for vapor permeance, it’s safe to say that cement backerboard is highly permeable.

    Manufacturers of cement backerboard generally recommend that a moisture barrier of some sort (WonderBoard calls for #15 felt or 4-mil polyethylene sheeting) be installed behind the backerboard when used in a wet location. According to some tile contractors, however, this is bad advice. “Plastic is a bad idea because you are nailing it on and putting holes in it,” says Tom Meehan. “When there is plastic, I’ve found mold behind the plastic. It locks any moisture behind it, and the moisture can’t dry.” If you want to waterproof the wall, Meehan recommends the use of a liquid-applied membrane such as Laticrete Hydro Ban or Mapei Mapelastic AquaDefense on top of the backerboard. For a steam shower, he prefers a sheet membrane from Kerdi or Noble.

    Ideally, this type of wall will be able to dry outward. If that isn't possible, at least make sure that flashing details are impeccable, so that wind-driven rain is prevented from entering the wall assembly.

  2. wjrobinson | | #2

    Redgard also Martin.

    As to Southern build, if you add rigid foam behind backer board that may help make the shower less able to collect outside moisture behind it from outside humid air condensing if that is what your question relates to.

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