I'm building a new tiled shower stall with cement board backer. I've read that a vapor barrier should be placed over the wall studs before the board is installed, because the tile/cement board combination is not waterproof. We have an acrylic shower receptor, the directions for which also suggest a vapor barrier. I plan to use six mil plastic sheeting for the vapor barrier.

That's all fine, but what happens to vapor/water that get behind the cement board and stops at the vapor barrier? It sounds like I could develop a lake - probably not a good thing. How should the vapor barrier be installed so that moisture will drain/evaporate. (These are interior walls.)

Thanks in advance for the help!

skiberger

Good question.

The Tile Council Handbook says a vapor barrier should be used when using cement board in wet areas (15lb roofing felt or 4 mil poly)

What you can do is set your shower pan first. What work well underneath that is "shower pan liner" sheet that is used in a mud bed application.
Set your pan first then place the vapor barrier on the walls and extend the barrier over the lip of the shower pan. Then install the cement board.