Bathroom Exterior Wall – Cold Climate
celkanawati
| Posted in Construction Techniques on
Hello,
Building a house in Montreal and have a question regarding the wall assembly of an exterior wall which happens to be a bathroom (shower and bathtub).
Brick cladding on the outside. The structure is 2×6 wood studs. The cavities will be filled with spray foam. For a level surface, furring strips will be installed on the 2×6.
The inside of the bathroom will be tiled and the backing will be water-proofed.
My question is whether which of the following makes sense:
A)
2×6
Furring strips
5/8 regular drywall
1/2 greenboard
Waterproofing membrane
Tile
B)
2×6
Furring strips
1/2 greenboard
Waterproofing membrane
Tile
I very much appreciate the feedback.
Charif
Replies
please no drywall. use tile backer board of some sort
Thank you!
Schluter Kerdi membrane is manufacturer-approved for installing over drywall so that's ok in itself but I'd still not do it. Cement board would be better even with the same waterproofing over it or you could use kerdi board which is water-proof and vapor retarding itself. There are other foam boards you could use.
What I would worry about is that spray foam you're planning to spray between the studs if it's closed cell. It will block all vapor so if you have a leak (and small ones can happen) or if you just have some small amount of vapor getting through the waterproofing, it will be trapped between the waterproofing and the insulation and rot the studs and mildew up the place. I would opt for open cell foam or (more likely) just use mineral wool which is cheaper, easy to install, and is more environmentally friendly, or so I hear.
I dont understand your concern with closed cell foam inthat application. Water getting into open cell or mineral wool will have an incredibly tough time f=drying to the outside and will probably cause the same rot you are worried about.
I prefer closed cell as a product as its superior in creating an air barrier andif its being used thoughout the rest of the house it would be problematic to change material just for the bathroom
Better to make sure your waterproofing beit red gaurd or kerdi is well applied and there are no leaks.
The concern is in creating a vapor barrier sandwich and it's a common one. You don't put waterproofing over tile backer and plastic behind it, just one or the other. I'm not imagining a gush of water getting through like leaking from an ice dam or something, just small seepage or even vapor getting by the 1st barrier. If that's all, it's got a good chance to dry. Probably more so with the mineral wool than the closed cell foam, that stuff is pretty airy. I've seen interior air get through it when it's uncovered and condense on wall sheathing behind it in the winter so I know vapor gets through it.
Changing material just for the bathroom would be a problem but I interpreted the original post to mean that they're getting foam in that wall alone, not to match the whole house. And I can't disagree with making sure the waterproofing is well applied with no leaks but mistakes happen so it's good to have robustness elsewhere and the vapor permeability of a lot of waterproofing is actually higher than you would expect according to the spec sheets. Some of them are mean to keep water out but some vapor still gets through. The manufacturer will not guarantee them for steam showers. I think redgard is actually one of those (or used to be). Others like aquadefense have to be applied to different thickness spec to be true vapor barriers.
Anyway, I don't build bathrooms for a living but there are a lot of people on the John Bridge forums that do: https://www.johnbridge.com/vbulletin/index.php That would be a good place to ask.