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Will exterior wall assembly still dry if vapor barriers are applied to BOTH sides?

Sal Lombardo | Posted in Energy Efficiency and Durability on

I constructed a home following the outsulation concept I learned from GBA. I have 1.5” exterior rigid XPS, caulked and taped on the exterior of the wall finished with 3 coat, real cement stucco applied over Keene dri-wall venting plane.  Interior to that are back-up vapor barriers, asphalt felt, Tyvek then 5/8” plywood sheathing, 2×6 stick framing, the fluffy pink stuff – fiberglass insulation, R-21, in the bays and then 5/8” sheetrock. Thus the wall is designed to dry to the interior.

In the master bathroom, one wall is a stone clad accent wall. Instead of Sheetrock,  I used 5/8” cement board, glued and screwed to the studs and plan to install stone veneer with thin set to the cement board. Since the bathroom is a very humid and moist area, I want to apply a roll-on liquid moisture barrier to the cement board first, like a Red Guard or Hydroban. Then install the stone and finally use a clear paint-on stone sealer to further stop moisture from entering the wall.  

Without the moisture barriers, I am concerned the moisture would be absorped by the porous stone, wicked onto the cement board and ultimately transmitted to the stud and fiberglass batts. But If I install said moisture barriers don’t I compromise the drying of the wall to the interior?

It is only one wall, on the second floor, 8 ft x 18 ft, has 2 windows and protected by oversized 38″ overhangs.

Would that wall still dry to the interior via adjacent walls and up into the attic space? Do I have to be concerned?


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  1. Matt F | | #1

    What zone are you in?

    Is this wall part of a shower? If it is not, no waterproofing should be needed.

    You could put Membrain behind the cement board. The cement board, sealed rock assembly may be similar permeability to drywall, but I don't have any numbers.

    If it were a shower wall on an exterior wall, some sort of interior rainscreen could be used to allow moisture redistribution around the moisture impermeabile section, but I can't say I have seen that done.

  2. Sal Lombardo | | #2

    Zone 5
    Not part of a shower wall.
    In my current bathroom, as I imagine in ALL bathrooms, long hot shower in the cold winter leaves condensation on all walls. The stone has to absorb this and wick it to the cement board. I will have an ERV pulling out of the bathroom to help mitigate moisture accumulation and facilitate drying.
    Thanks for your input either way.

  3. Jon R | | #3

    Sounds like you will have no < .1 perm vapor barriers. More important than permeability is ex-filtration and indoor humidity. A high CFM exhaust fan (during and a little after showering) will help with both.

    I also use an infrared lamp while showering - it warms the walls and reduces condensation. Exactly how much isn't something I've tested.

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