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Drying Potential of Bathroom Walls

maxwell_mcgee | Posted in Green Building Techniques on

Hi all,

Planning out a new build in CZ 5/6 which will include a Zip-R wall assembly. It’s designed to dry to the inside (given the poly-iso plus the OSB plus the built in membrane on the Zip panels won’t allow drying to the outside), so will have an variable-permeance membrane (likely Intello) on the inside to allow the wall to dry out if it gets wet.

Now, I’ve seen the pictures online of what happens behind vinyl wallpaper in air conditioned buildings in the summer months where walls are designed to dry to the inside. So I’ve committed to not putting up vinyl wallpaper on any exterior walls.

My question though — my layout has bathrooms on the exterior of the building (we designed it that way to allow for natural light in the bathrooms). How does a wall dry to the inside if there are showers/tubs/mirrors/tile walls/etc. touching the wall?

And if the answer is that it doesn’t dry to the inside, does that mean that I’m setting myself up for a major mold problem behind the bathroom walls unless I can redesign my walls to dry to the outside?

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  1. Patrick_OSullivan | | #1

    > And if the answer is that it doesn’t dry to the inside, does that mean that I’m setting myself up for a major mold problem behind the bathroom walls unless I can redesign my walls to dry to the outside?

    In what scenario do you envision these walls taking on enough moisture such that they would need to dry out?

  2. maxwell_mcgee | | #2

    My understanding is that one of the basic tenets of building science is to assume every assembly will get wet and to think through the process through which it'll dry.

  3. Debra | | #3

    Maxwell, I've wondered the same thing myself. It's very common for bathrooms to be located on an exterior wall, with the impermeable shower walls up against them. I *think* this is a bigger issue in hot, humid climates where the summer vapor drive is from the outside to the inside, and the humidity could condense on cool, impermeable interior surfaces (hence, no vinyl wallpaper, etc). But is that still an issue in cold climates like yours, especially with an exterior wall with lower permeability?

  4. Expert Member
    Akos | | #4

    I'm in zone 5 where code has been 2x6+r5 rigid with interior poly. Bathrooms are placed on outside walls all the time without issues. What you have to watch is air barrier continuity especially behind a tub against outside wall. The VR should be continuous there and sealed to the bottom plate. This is something you have to keep an eye on as some contractors think the poly is in their way and simply cut the bottom half out.

    Also avoid windows in showers, even well detailed, that is just asking for trouble.

    1. maxwell_mcgee | | #8

      So if I understand it correctly, the logic is here is basically that we keep the interior moisture from the bathroom from hitting the wall cavity and then hope for the best?

      But if this is a good strategy, then why do we often see the crazy mold issues on the backside of vinyl wallpaper or 6 mil poly? Is it that the mold is behind the shower too, we're just not often opening up the wall back there in order to see it?

  5. andyfrog | | #5

    I can't vouch for the cost, constructability, or effectiveness of this assembly, but this is apparently a regular detail in Finland that permits drying. The blue dashed line isn't labeled in the drawing, but it's waterproofing. Edit: it is labeled.

    1. maxwell_mcgee | | #6

      Am I interpreting that correctly?

      That’s basically a rain screen gap on the inside of the wall separating the shower tile from the drywall?

      1. andyfrog | | #7

        There are similarities but I think of it more like a freestanding tub with a cover inside a room. Some of the details like the waterproofing and capillary breaks at the bottom are notable.

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