New cold climate Bathroom: Use Vapor retarder?
New full bath project. Its situated in such a way that it does not share an exterior wall of the house so no windows and none of the considerations about exterior wall insuulation concerns in zone 5. We are installing a 110 CFM (or larger) exhaust fan with humidity sensor. The 2×4 partition walls around the bathroom will be insulated to deaden sound with rock wool batts. All drywall is to be green board (or purple…whatever color the moisture/mold resistant it is now) and will be primed and painted with latex designed for baths. The space above this bathroom is an unconditioned attic insulated with un-faced fiberglass batts that gets the living space ceiling up above the required R49.
I want to make sure I don’t get a bunch of moisture in the walls and up into the insulation above. I moved up here from the deep south where people use poly as a barrier but in this cold climate I know that is a no-no. Was considering using a semi-permiable vapor retarder like Certainteed’s MemBrain. I have not used it before and wanted to check to see of this is the right plan of attack.
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Make sure your new walls are sealed, so that you are not leaving air pathways into the attic. You also should pay attention that any penetrations into the attic space are sealed. This includes electric, heating and plumbing.
The walls will be ok with your exhaust fan, and proper paint. If the fan is off, or in other conditions, condensation will happen on the bathroom surface of the wall, and evaporate as air circulates.
You can use a primer formulated to be a vapor barrier (on the bathroom side) and call it a day. If you avoid such a primer on the other side of these new walls, any moisture that does make its way into the wall will migrate back into the rest of the house.
Insulation gets into trouble when air flows, and the temperature drops below the dew point of that moving air inside the insulation
You will want to pay attention to the seal at the exhaust fan, and use insulated duct to get through the unheated attic to the outside (to avoid massive amounts of condensation that can build up and leak from the duct.) (do not just dump it in the unheated attic.)
What kind of shower/tub enclosure do you plan to include?
Its a 3x5 tiled shower with a partial glass sliding door. I am using ditra and hardy backer for the tiling. Entire bathroom is 10x5. The exhaust vent is centered over the shower pan and I am using insulated venting for the 6-7 foot run out the side of the building. The planned insulation in the walls around this bathroom is not for R-value but for sound reduction.
All penetrations into walls and ceiling will be sealed with foam to eliminate air pathways. I rarely glue and screw vertical drywall due to expansion and contraction but may plan to do so at the bottom plate to eliminate those possible pathways as well.
Exhaust fan.
Proper gap at door so it does what it’s designed for.