In this answer it was stated that:
I would recommend paperless drywall over cement board for the tub surround, as well -- gypsum board of all types is vapor open (around 50 perms) while serving as an air barrier, allowing drying to the inside without letting a ton of humid air into the wall cavity, while cement board is a fairly severe vapor retarder at 4-8 perms, which puts more pressure on the ability of your tub surround to keep water out of the wall to begin with.
What are the differences between a "paperless drywall" and regular drywall (I've never seen "paperless drywall" at my local big box store, so would have to expand my life experiences if I were to go looking for it) and cement board in terms of their use as a backer behind a tiled tub/shower surround?
Would the fact that one of the surround walls in the question that answer was provided on was on an exterior wall make any difference in the application of this advice?
It has been my understanding that cement board (with a waterproofing membrane of some sort) was the appropriate backer for a tile surround, and this now has me questioning that assumption. Additionally, I would think that a water proof membrane for the shower surround would also render it vapor proof, but I've been wrong before.