Tiles/showers on exterior bathroom walls — mold risk?
I’ve seen a few articles that talk about the risk of placing non-vapor permeable items against exterior walls in hot/humid climates.
The risk is that in the summer months, the interior surfaces are cooler/dryer than the exterior and so vapor drive is inward, and so vapour will condense on the back of the non-vapor permeable item.
So for example, don’t put up vinyl wallpaper, mirrors that are right up against the wall, etc. as vapor will get trapped and mold will form in behind there.
But what about showers that have tile walls, or bathrooms that have tiled walls from floor to ceiling. Presumably most of these ceramic/porcelain tiles are vapor impermeable.
Given how many houses have bathrooms on exterior walls (a window in a bathroom is a nice feature!). Is this a big risk?
Building Science Corp. explains some of the issues here.
https://buildingscience.com/documents/building-science-insights/bsi-130-hotel-mold
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Answering my own question with a question...
But is there a product similar to Schluter's Ditra mat that could act as a backerboard to wall tile in a shower or any other exterior-facing wall? While the normal uncoupling feature of a backer board seems valuable for any type of tile installation, the Ditra-style mat also seems like it would basically create a dimple mat or interior rain-screen assembly. Wouldn't that also address the vapour-condensation concern?
As far as I can tell, Ditra doesn't seem designed for wall tile applications, but are there other manufacturers who make something similar that is?
https://sccpublic.s3-external-1.amazonaws.com/sys-master/images/h73/he1/9084108931102/ditraheat-duo.jpg
In my area (zone 5 with warm/humid summers) tiles are regularly installed on outside walls with 6mil poly behind the drywall/cement board without issues. About the only time I've seen problems is with improver waterproofing where a lot of water makes its way into the wall.
If you are still worried, you can the Kerdi insulated tile backer. These can be had in 3/4 and 1" thickness which should be plenty for condensation control in warmer climates. Since the condensing temperature plane is now somewhere inside the rigid insulation, moisture can't get to it so you won't get any condensation.
You're in Southern Ontario, right?
Yup.
I've seen plenty of old bathrooms with mold in the walls and wondered about this. Here's an idea for an approach in new builds that might solve the problem. Let me know if you see any obvious flaws. It seems to me it would be fairly economical.
1. Assume we have a shower in a corner against an outside wall and the adjacent wall surfaces outside the shower are not tiled. They would be vapor permeable.
2. After framing and insulating, before applying interior drywall, cement board, etc., furr out the interior face of the stud walls with horizontal 1x strapping. This would make all the stud bays in the wall section communicating.
3. Any moisture accumulating in the portion covered by the tile shower would be able to evaporate via the adjacent non-tiled bays.
There would be a loss of 3/4" of floor space. So that would need to be planned for if you have fixtures with tight clearances. But otherwise I see no issues.