GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

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?

Thanks

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  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.

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |