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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.

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To understand paperless drywall, you need to understand paper drywall

Normal drywall (sheetrock, plasterboard) consists of a gypsum-and-fiber core that is pressed between sheets of heavy (construction) paper and then dried together. This provides an inexpensive and easily producible product with predictable properties, but has the downside that the results, while vapor open (letting water vapor through), are water sensitive.

In order to provide a noncombustible, lightweight exterior sheathing product to go with steel studs, drywall manufacturers started sandwiching a treated drywall core between two layers of fiberglass, which is the "paperless drywall" (more correctly called glass faced gypsum sheathing) that I was referring to in the linked post. It maintains the vapor open properties of regular drywall, as well as most of its other good traits, while being much more moisture resistant, as there's no paper there for mold to snack on.

Cement board, on the other hand, consists of cementitious (pozzolanic) materials, a fibrous or flake binder, and miscellaneous additives that add water resistance and structural strength, all baked together into sheets. This makes it a much more water resistant material than regular drywall, but it's also much heavier, and much less vapor permeable.

As to the shower problem...

In the context of shower systems, generally speaking what's fitted is a waterproofing system, designed to divert bulk water back into the shower drain, where it belongs. Any vapor barrier properties it will have are treated as incidental and not warranted, unless you're designing a steam shower setup, where a vapor barrier is actually needed.

That said, many shower systems do have some vapor/air barrier behavior to them, particularly higher quality setups that use fluid applied (RedGard) or continuous sealed membrane (Noble CPE) waterproofing. These probably should not be run up to an exterior wall to begin with, but if you must have them on an exterior wall, you'll want to terminate them short of the ceiling (normal) and make sure that water vapor laden air can escape the wall cavity to the inside, below and behind the shower enclosure, as well. This means that there's at least some path out for moisture that gets behind the enclosure, as the last thing you want to create is a moisture trap, where water vapor can't get out by hook or by crook.

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