I'm replacing a standard tub/shower that had tiled walls, with a free standing 3 piece tub/shower (tub / sides / top). Since it is free standing (secured only at the front) and because of its shape, there will be a substantial air cavity above and behind the shower.

I had to tear out all the drywall and re-insulate (outside wall), due to mold, etc. from previous leaks in the tiles.

Since none of the back or side walls or ceiling will be visible with the new shower, what is the appropriate type of covering to put on these surfaces (beyond insulation and vapor barrier) ? Do they need to be moisture tight ?

Also, how strongly should I attempt to make the cavity moisture tight ( given that it is not likely to be 100 %)? Is it better to allow some level of air flow to try to address moisture that will find its way into the cavity from the high humidity of the room during showers (assuming that the more air flow is allowed, the more humidity will enter the cavity, while restricting any air flow will not necessarily stop all moisture) ?

Thanks

Mike