GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X 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 a smart vapor retarder offer enough moisture protection to be used for bathroom walls?

Dave Brooks | Posted in Energy Efficiency and Durability on

We’re planning to use MemBrain for an interior vapor retarder throughout our two story house currently under construction. The wall assembly is designed to dry to the interior. We want to limit moisture diffusion into the walls for the short duration of high humidity during showers while allowing walls to dry if sheathing accumulates moisture during heating season. Will MemBrain offer enough moisture protection to be used for outside bathroom walls?

Wall assembly in Zone 5:
Ox Engineered Products SIS for sheathing with 1″ Polyiso R 5.5, taped and sealed to function as air and vapor barrier, 2 x 4 filled with Roxul mineral wool to R-15, 1/2″ drywall with two coats of latex.

We also have some ReWall Essential board we can use. This offers great mold resistance, but I don’t know if it will be too vapor tight. I’ve wondered about using it instead of drywall for 1/2 the wall over one-piece fiberglass showers and tubs, with drywall below behind the shower wall.

We’d appreciate recommendations.

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.


  1. Expert Member
    Dana Dorsett | | #1

    You'll be fine in zone 5 with that stackup.

    The interior face of the SIS is moisture tolerant, and the R5 on polyiso on the exterior would be sufficient for dew point control in a batt-insulated 2x4 wall in a zone 5 location even if it were moisture-susceptible OSB or plywood. Condensation events will never be long enough in duration for moisture/frost to build up inside the stud cavity, and the cold side will average well above freezing over the winter.

    The MemBrain will limit the rate of moisture accumulation, but would allow it to dry as quickly as the interior finish surface paint/tile/whatever allows. A couple coats of latex would run close to 5 perms, but except during showering events the winter time permeance of the MemBrain will be less than 1 perm. If moisture inside the cavity ever increases to where the cavity-air is above 40% RH at the temperature of the drywall, the MemBrain will be more vapor open than the paint.

    ReWall Essential is 100% cellulose. They don't specify the vapor permeance, but like most fiber-board materials it has to be VERY vapor permeable, and would need to be coated with something (like latex paint) to adjust it down to even Class-III vapor retardency.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |