Bathroom renovation – exterior wall construction?
Helping my brother with a bathroom renovation at his new (first) house.
The bathroom has been completely gutted, and has one exterior wall. The house is in NJ/Zone 5. Exterior wall is 2×4, with plywood sheathing. Not sure of the sheathing thickness or what lies beyond it (housewrap, rigid insulation, etc.), but the house has aluminum siding. The house was built in 1955, so it’s probably just felt paper, unless it has been upgraded.
Over the weekend, my brother installed rock wool insulation in the exterior stud bays. The plan is to ultimately install drywall and cement board, followed by paint and tile, respectively.
Should we put anything between the insulation and drywall/cement board? We’ve considered:
– Plastic sheeting/vapor barrier to prevent moisture generated in the bathroom from getting to the studs/insulation
– 1/2″ XPS foam to serve as a vapor barrier, plus reduce thermal bridging and provide some (very limited) increased insulation
– Nothing
– Something else
What do you think?
Thanks.
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Replies
Matt,
This is a fairly forgiving wall assembly. As long as you pay attention to airtightness (always a good idea), I don't see a lot of risk here.
Because it's a room that might be humid, you might want to install a smart vapor retarder (a product like MemBrain) between the studs and the drywall. MemBrain would be less problematic than polyethylene during the summer, when the house might be air conditioned.
If you want to install a 1/2-inch-thick layer of rigid foam, you certainly can.
In any case, you need some type of code-compliant interior vapor retarder. If nothing else, you need to install vapor-retarder paint.
Thanks, Martin.
Can we install MemBrain AND XPS?
If so, which order? I would think MemBrain over the insulation, then XPS.
Thanks.
You could install both, but the additional benefit is small to non-existent. The foam is about 2 perms, the low end of Class-III vapor retardency, and you probably don't really need ANY sort of interior vapor retarder tighter than standard latex paint.
Find out what's between the plywood and the aluminum siding. With a hole saw drill out a 1-2" plug to get a really good look at it. (Replace the plug, sealed with polyurethane caulk when you're done.) If it's just #15 felt (tar paper), you're overthinking this. AL siding is inherently back-ventilated and your stackup would meet the IRC's definition of "Vented cladding over wood structural panels" in chapter 7, which in your climate zone means it doesn't need a specific interior side vapor retarder- even latex paint would be sufficient. see:
https://up.codes/viewer/general/int_residential_code_2015/chapter/7#R702.7.1
That said, if going with full sheets of foam you'd get more bang per half-inch out of foil-faced polyiso , which is a LOT greener than XPS, and would also outperform it on a thermal basis. Foil facers are easy to air seal with foil tape, and you'll get R3 out of it rather than R2.5 for XPS, falling to R2.1 over time, as the climate damaging HFC blowing agents bleed out.
If you can spare 2" of interior space edge strips for the studs made of 1.5" polyiso (R9) and half-inch CDX ripped to 1.5" as a nailer for the wallboard, with R23 rock wool in the cavity will nearly double the performance, and would even be beyond current code-minimum. With that stackup MemBrain under the wallboard might be "worth it", though still not necessary to meet code.