I'm using kerdi membrane for my shower area with cement backerboard. Should I still add a layer of plastic vapor barrier behind the backerboard. Normally that's required to prevent the studs and insulations from wicking up water from the cement board, but I've seen discussion about water being trapped in the backerboard if the board is lined with Kerdi on one side and water vapor barrier on the other side. What about the rest of the bathroom, vapor barrier behind greenboards? Any definitive answers?
Kerdi is your barrier if properly installed it will stop water from migrating. I don’t recommend a barrier behind green board it is better than standard sheetrock for moisture but sealing it may cause more problems than can be gained. A good quality exhaust fan with the lowest noise or Sones will do be the best way to go.