Wall construction and interior finish

[bluedog225]

Good morning,

I’m trying to figure out how to allow my walls to dry to the interior in a bathroom where I plan to finish the walls with something like tile that will form a moisture barrier. Located in central Texas.

The exterior walls are sheet metal with a rain screen, Tyvek, taped foil-faced polyiso foam, over 3/4 plywood. As such, the will be no drying to the exterior.

I vaguely recall, hearing stories about schools putting up whiteboards (or black boards) on the wall and ending up with mold on the back side because of trapped moisture in the wall. I can see a similar situation if I put concrete board and tile up on an interior wall in the bathroom area.

Is there a traditional way to avoid this? I can get a little creative and put some type of self ventilating system with perforations top and bottom on the wall to allow airflow behind the tile. Or I can abandon my ideas of using moisture barrier like hard tile and instead finish off the bathroom walls with something like shiplap.

Thanks

 

[tmp]

Why do you need to dry to the inside? Where do you expect the moisture to come from? Sounds like you have good water management on the exterior. Your foil faced polyiso will act as a vapor barrier on the exterior.

Certainly this assembly is done all the time with no issue. I’d be inclined to forget about the drying potential in this room and just go for the tile if that’s what you want.

 

[bluedog225]

I’m pretty sure it has to dry to one side or the other.

 

[tmp]

Bear with me because my understanding of vapor movement is somewhat limited.

Absent of bulk water (ie rain) getting in the wall you’re really only concerned with water vapor. Water vapor would move hot to cold, so the vapor drive in Texas will typically be from the outside to the inside. Your foil faced insulation is acting as a vapor barrier on the outside of the building, so you won’t be getting vapor moving in from the outside. Your interior wall finishes being vapor permeable allow vapor into the wall, but also allow it to dry with the same mechanism.

Assuming you’re waterproofing the walls behind the tile, I don’t see how water vapor would be making its way into the wall, and I don’t see the need to facilitate drying.

Take this with a big grain of salt, I have limited knowledge here. Post this over on the Green Building Advisor or Fine Homebuilding forums - that’s where you’ll find the experts.

 

[manwithtools]

Bathroom exhaust fan should take care of any internal moisture, no? I think you are overthinking this.

 

[CraigStu]

We have a heat light over the shower and an exhaust fan. We turn the fan on at the wall switch but it also comes on automatically if we forget. But in my mind it auto-on's at too high humidity. I'd look into an auto-on fan w/ some adjustment available for the humidity level that switches it on.