I cannot seem to find a straight answer on this topic so hopefully someone here can help. I am getting ready to gut my upstairs bathroom. The 3 walls where the tub is is located are all interior walls but there is one exterior wall in the bathroom. The intent is to install an acrylic tub w/3-piece acrylic/fiberglass surround. I have done quite a bit of reading and realize if I was going to have the walls above the tub tiled I would require either a vapor barrier behind the cement board or a waterproofing membrane system like Kerdi or RedGuard between the cement board and tile. However in this case since it is all fiberglass/plastic - whatever they are made out of - and non pourous/waterproof, is the 6mil poly sheeting required or recommended to be installed over the studs from the top plate down to the lip of the tub? I've looked at NJ builders code and have no clue where to find the answer. It is also my intent that any drywall that gets installed will be the new purple Gold Bond XP drywall that is both mold and mildew resistant. The goal here is to keep everything dry and mold/mildew free. Being the two short and one long wall of the tub are all interior walls and being the surround is (I am assuming) waterproof (seams would be caulked) is the vapor barrier necessary? Thanks in advance for your input.

 

I'm not an expert but I've torn out 4 tubs with fiberglass surrounds. None had a vapor barrier behind and none had any water or mold issues.

I believe that if the bath has adequate ventilation and is maintained properly there will not be any mold problems.