It won't get out. I've been researching redoing my tub surround and it appears that most people use 15 pound tar paper. I think the tar paper will eventually let the moisture through if any does get in, and it will be at a slow enough rate that it doesn't hurt anything. The only place they seem to use poly is if it is a outside wall and the concern there is you don't want moisture to penetrate from the outside. I'm no expert, but that's the way I read things right now.

We are in the process of re-tiling our bathroom and I have completed a complete gut of the existing shower space...well, almost a complete gut. I removed the existing tiled shower walls by cutting into the greenboard (I think) about 8 inches above the top row of tiles, and ripping out the entire wall of tiles. This exposed the studs behind...and also exposed the fact that there was nothing else there. No insulation (there aren't any exterior walls), no vapor barrier...nothing. Just a bunch of rusty nails! So after searching this forum for a bit, I learned about installing insulation between the studs, and the vapor barrier (6 mil poly) between the studs and the cement board. However, my question is the space above the gutted area that I left untouched since it won't be tiled and therefore did not need to have the greenboard removed (there's about 2 ft to the ceiling). Can I just add insulation/poly to the area that will be covered by cement board/tile, or will that just create more moisture issues above this area? Should I rip the remaining greenboard out to the ceiling and put insulation/poly/cement board all the way to the ceiling even if we don't tile the entire area? Or should I just install the cement backer board directly to the studs (no insulation, no poly) and take my chances? Any advice would be greatly appreciated. Thanks!

This topic comes up often enough on these forums... My personal opinion is this... There's not a lot of water flowing behind your tile layer, there may be some moisture that will penetrate the grout. That moisture will evaporate back through the grout, the plastic is there to keep it from evaporating into the stud cavity. If you have large amounts of water flowing behind your tile, you have serious problems. Now the caulk is absolutely necessary. A shower head aimed towards the tub/tile joint will create enough pressure to push the water under the tile, over the tub lip and into the ceiling below. My $0.02

Hire a plumber to properly insert an inflatable test plug and fill the pan. You will very quickly FIND cause. It is something you can do yourself but to be thourough the test plug should be inserted down the stand pipe Below the drains flange. If you do find a failed connection or otherwise leaking shower pan, the best way to avoid this in the future is with a flood tested shower pan. Many tilers even today skip and scough at Flood testing their own work in leu of hurrying....