There is discussion of vapor barrier behind a shower at this question, and many recommend not to have a vapor barrier behind the backer board if you are using a membrane on top of the backer, as it will create a "moisture sandwich" which cannot dry out.
HOWEVER: I am putting the shower against an exterior wall, I am reluctant to follow the above advice as it will not maintain a continuous vapor barrier for the exterior wall; that is, I cannot join the shower membrane to the existing exterior wall's barrier.
What should I do? I am considering keeping the existing vapor barrier in place and leaving a gap in the membrane at the top and bottom, so the backer board can breathe to the inside of the house. Will that prevent a moisture sandwich? Will it cause other problems?
Part II of my question: I am using a pre-fab pan and just tiling the walls, not pouring or tiling the pan. The instructions with the pre-fab show using a silicone bead to seal the lip of the pan to the wall lining (vapor barrier I guess) BEHIND the backer board, and no membrane is shown in front of the backer. So another reason I like leaving the bottom of the membrane open is that I don't see how to connect it to the shower pan lip in this configuration. But if I change my mind, how is the membrane supposed to connect to the shower pan?