Will a smart vapor retarder offer enough moisture protection to be used for bathroom walls?
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 Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
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.