Exterior wall behind tub/shower pan
CZ 3A/4A
2001 gut reno
I’ve looked at other answers to similar questions and am not entirely sure if I’m understanding correctly, so I would appreciate some additional help with this!
In our guest bathroom, we will have an acrylic tub with tile flanges on an exterior wall and will be doing a tile surround above it. I understand that we need to insulate and air seal behind the exterior wall of the tub.
Where I’m getting confused is:
1. What material is best to use for the rigid air barrier (unsealed Durock, plywood, OSB). My contractor mentioned foam board, but I’m not sure about that.
2. How thin/thick should the air barrier be?
3.. How far up should it go – Should it extend from the floor to the ceiling and then the tile backer is installed on top of it? Should it go from the floor to just above the top of the tile flange? Should it stop flush with the top of the flange?
4. If only going to the top of the flange or slightly above, how should the tile backer (most likely cement board) be installed so that it overlaps the tile flange? Do we need shims?
We will be installing a shower pan in the primary bath using the same principles (just on a smaller scale obviously).
Thanks so much!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Seeing as you are in climate zone 3 or 4, building code will allow you to use a class II vapor retarder on exterior walls. These membranes can be installed in a way to be an effective air barrier. The most economical product I believe is Certainteed's Membrane. A quality tape or an acoustical sealant can help in making the air barrier continuous at seams and joints.
This membrane would go from floor to ceiling. Once it is in place, the tub can be set and the wall surface installed.
I wrote an article a while back that covers some of the basics when working with air/vapor barrier membranes: https://www.greenbuildingadvisor.com/article/installing-an-interior-air-barrier
Hope this helps,
Randy
What about in situations where something like that isn't readily available? I'd have to order it, and we don't have the time to wait unfortunately.
I've seen other comments and articles that mention using a rigid barrier is best for this. Is that not the case? Or does it just depend on climate zone?
If rigid is better overall, what material is best and does that still get installed top to bottom? Or is it possible to just install behind the tub and have the cement board and tile act as the air barrier above it (images attached)?
Thank you!
Personally, I'd prefer to see a continuous product in place before the tub is installed. Trying to get a transition between the area below and above the tub using two different applications can be difficult to make a continuous air seal. You could use a thin wood sheet good, like 1/4" or 3/8" plywood and seal the seams and edges using a tape or sealant, install the tub, then cover the plywood with the tile backer. If you use a cement board as a tile backer, the cement board needs to be treated to assure it is water resistant. We've used both roll on waterproofing "paint" products like Redgard and membranes designed for tile applications (Schluter Kerdi).
We're going to go with a 1/4" cement board from bottom to top plate to create the continuous air seal. What products are best for sealing the edges? I'm assuming we will just use regular cement board tape for any seams if needed. Is there anything specific that would work best for the edges of the board to seal between it and the framing?
What's the best way to attach the cement board to the plywood to ensure we aren't leaving air gaps for moisture to settle in? Also, wouldn't attaching it (and installing the tub for that matter) to the plywood compromise the air barrier since you'll essentially be poking holes in it? Is there a way to combat that?
Thank you!
Both the tub and cement board will be fastened through the plywood, directly into the wall framing. The plywood is only an air barrier, not as an attachment point for the tub or wall finishing material.
There are other products that will work as the air barrier, the rigid insulation idea your contractor had would work as long as it is sealed to assure it is airtight. If that is the product used, I would keep the insulation thin, 1/2".
I see. I guess I thought adding holes anywhere in the air barrier (even at the wall framing) would essentially pierce the barrier since those holes/fasteners won't be sealed.
Is it possible/reasonable to separate the area behind the tub from the rest of the stud bay by installing solid blocking in the stud bays with spray foam on all seams?
Or does the entire stud bay need to be included in one singular air barrier?
Thanks!
tjones,
Every air barrier gets fastened mechanically, and those fasteners have to penetrate it to work. Poly, house-wrap, or membranes get stapled. Exterior sheathing gets nailed. As long as they go into a solid backing there is no or only negligible effect on air-tightness.
Fasteners work by friction. If there was sufficient space between them and the hole they have created for air to leak they would come loose.
Thanks, Malcolm! As far as fastening cement board to plywood, is there any concern with there being air gaps between the two for moisture to settle in? We've seen where moisture can grow behind bathroom mirrors for instance because it wasn't fully adhered to drywall
tjones,
The plywood is vapour-open. Any moisture that makes its way through the cement board will be taken up by the plywood by adsorption, and diffuse though into the wall.
As Randy mentioned, you do need to waterproof the cement board, or use a membrane behind it.
Will it be able to dry in the wall? To me, vapor diffusing into the wall sounds like a problem.
Yes, we will waterproof the cement board. My concern was more about general humidity and vapor in the bathroom making its way behind the cement board and causing mold in between the cement board and plywood. I'm thinking about it the same way if you don't fully adhere tile, you run the risk of humidity getting in those air pockets and causing mold and moisture issues. Maybe I'm not thinking about it in the right way, though.
Water vapor moving into and out of wall assemblies usually isn't much of a problem unless it condenses. Changing phase from a gas to a liquid is problematic. Bulk water leaks are more worrisome. Water vapor moving through building materials by way of diffusion is very slow, if you can address humidity in the bathroom by using a bath fan, the risk will be reduced.
Deleted
Deleted
We're going to go with a 1/4" cement board from bottom to top plate to create the continuous air seal. What products are best for sealing the edges? I'm assuming we will just use regular cement board tape and thinset for any seams if needed. Is there anything specific that would work best for the edges of the board to seal between it and the framing? Would ChemLink M1 work well?
We have leftover siga wigluv. Would that work? Or I think we may have some extra fentrim as well