Q: As construction techniques and materials have advanced, we have gone from simple moisture-resistant drywall or cement board to plastic (polymer-based) bath and shower enclosure systems by Schluter, Mapei, Wedi, and the like. With these systems, do we need to start thinking of the vapor barrier and insulation behind these areas differently?
A: Kohta Ueno, a principal of the Building Science Corporation, responds: For the most part, having an impermeable material on the inside is typically not a fundamental problem. In standard construction in a cooler climate, you typically put your less vapor-permeable material on the inside, whether it’s polyethylene sheeting or a variable-perm retarder, such as Membrane or Intello. The downside of adding an impermeable layer on the inside is that you’ve reduced the amount of drying inward through the wall. So, if, for instance, you have a catastrophic water leak like a constantly leaking window in your shower stall, your wall has less of a chance to dry out.
In a shower, you have a lot of warm, moist air building up on the shower side and, in a cold climate, you have a strong vapor drive that wants to draw moisture toward the exterior. If warm, moist air leaks or diffuses into the framing cavity, it would likely condense on the inside face of the exterior wall sheathing. Fortunately, all the care and attention needed to install a shower system to be watertight also helps shuts down air leaks. Plus, the mechanical ventilation added to most shower areas helps exhaust the moisture and lightens the interior moisture load. For these reasons, failure (mold growth or rot) tends to be uncommon.
Shower enclosure membranes like Schluter’s Kerdi, or Wedi’s Subliner and the like, are not as vapor tight as, say, a sheet of plastic. I could see trouble in a hot, humid climate if you had a catastrophic air leak though the exterior into a shower-area stud bay. Inward airflow and condensation on the back surface of that enclosure membrane could become a problem. That’s why putting polyethylene vapor barriers or vinyl wallpaper in a wall in Georgia or Alabama is a terrible idea, given all the air conditioning needed in those climates.
The other possible failure in any climate zone is when you have a brick veneer wall with inward vapor drive. Brick soaks up water like a sponge. When the sun hits the brick, vapor moves both inward and outward (see illustration above), but the vapor drive is blocked by the waterproofing system used on the shower walls. Theoretically, this could be a problem if the vapor condenses on the back of the shower enclosure. But in practice, it is extremely rare that problems develop—perhaps if the shower wall was completely impermeable, as you might have for a steam shower enclosure. But again, reported failures are rare.
