[MGBROWN]

I have just gutted my master bath and will be retiling my tub/shower surround. It is on an exterior wall so the alcove is surrounded by insulation, which I will be replacing with newer stuff. How do I handle the vapor barrier issue? I will be installing insulation with the attached vapor barrier. I plan to put up cement board above the bath tub and tile accordingly. The ceiling above the tub will have regular drywall put on it and a vent fan will be installed. Do I need to put a vapor barrier behind the cement board and have it overlap over the tub flange? What about the ceiling? If a barrier is needed what is the best type. What do I do with the rest of the exterior wall that is not in the tub, do I use Green board on it with a vapor barrier and what about the two interior walls are barriers need on these as well.
Any help would be appreciated.