Confused about Shower Walls & Vapor Barriers
I’m in the process of making a DIY shower. I’m in climate zone 5A (central NH). So far, I have installed a 6 mil vapor barrier/retardant on two of the three walls where there’s insulation in the wall cavities. Permabase cement board is used on all walls and the ceiling. The joints have been taped and mudded with thinset mortar. I have also installed a pre-slope and Oatey pan liner on top of the pre-slope. I’m about to mud the bed with dry pack.
I would like to use a liquid membrane on the inside of the cement board, but am concerned about the issue of creating “vapor sandwich.”. However, I’ve read in a few places that if I use a liquid membrane on the inside of the cement board that it’s not as much of an issue with cement board because it will not degrade or deteriorate if it’s wet. I’d like to use the liquid membrane mainly because I now realize that I made a mistake in lapping the Oatey pan liner over the 6 mil vapor barrier instead of the vapor barrier lapping over the pan liner to direct water into the pan.
My questions are:
1.) How much of a problem did I create with having the sheet vapor barrier behind the pan liner?
2.) How much of a issue is it to use the liquid membrane on the inside of the cement board?
3.) If it’s a significant risk, would it be wiser to only use the liquid membrane on the one wall that does not have a sheet vapor barrier? What about the ceiling that has faced fiberglass insulation above it?
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Replies
Showers are always tough because they have the potential to get wet from both sides. I'm not a big fan of 6 mil poly vapor barriers except in very cold climates (zones 7 &8), but using it in Z5 is not a deal killer. Using a liquid membrane on the cement board is probably OK too. Yes, you create a vapor sandwich and the cement board might not dry very well. But the tile itself is a very good vapor barrier, so if water is getting to the cement board, it's not going to dry very fast anyhow. You can also take a look at the liquid you're using. Some of them are relatively vapor-open (compared to the tile, anyhow), so that's not going to be the limiting material for drying. If there is any question at all about how the materials are lapped behind the tile, I would go ahead and use liquid membrane on the cement board before applying tile. The tile is going to leak some and you don't want that water in the wall cavity. Worst case, you redo the tile in 10 years rather than replacing the entire wall.
Thanks for the reply Pete. Appreciate it.