"Greenboard" is no longer accepted as a suitable substrate for showers. Anyone who says differently is definitely living in the "Horse & Buggy" era. Do NOT let him back into your home! You either need to put a poly liner on the studs with a cement board over it or NO poly on the studs and a SAM (Surface Applied Membrane) on the outside of the walls. This membrane must pass ASTM 118.10 criteria. Anything else is a recipe for failure. Schluter's "Kerdi" is one good example...another is Laticrete's "Hydroban." There are others, but these are cutting edge technology....assuming you want it to last more than two years. If you dont get, or need more information, try going over to John Bridge dot com and post your project on the Tile/Advice Forum.

First, the tuck tape. Look for "sheathing tape". It's usually sold in the box stores, sometimes it's red colored. You could use Tyvek tape as well. The vapor barrier is a bit tougher to diagnose. Your exterior foam, "expanded polystyrene" is usually quite leaky with regards to moisture transmission and is not a strong vapor retarder. "Extruded polystyrene" is a much tighter product, and it could indeed be a vapor barrier, probably a Class II barrier. If the exterior foam is indeed continuous on the walls and well detailed, then it might be best to omit the interior poly and just use the Roxul. Your walls may take in some vapor over time, but the vapor should also transmiss back into the bathroom during drying cycles. The biggest problems occur when you have two barriers and one or both are poorly detailed. Over time moisture gets in but has difficulty getting out. Thus the "sandwich" that you referenced. The "imperfect" in your system is the exterior foam. I'm a proponent of foam, but only if it is the right type and the proper thickness. With your exterior foam probably being a Class II on the exterior side of your wall, your best bet is probably to omit the interior poly and simply use other methods on the interior walls. You can use something like Hydroban in your shower (if the shower is on an exterior wall), it's a waterproof barrier but does allow vapor to transmiss through it. For the remainder of your walls, simply use two coats of latex paint over your interior drywall. Latex paint is classified as a Class III vapor retarder. As such, it'll slow moisture vapor transmission that wants to get into your wall, but if any does, it'll allow your wall cavities to dry out during a drying cycle. It's an imperfect solution for an imperfect situation. I'd be completely confident in my reply were your exterior foam thicker than it is and if I knew it was well-detailed. My concern is the interior side of the foam reaching dew point temperatures in winter and any moisture vapor within the framing cavity condensing on the interior surface of the foam, between the foam and the sheathing. That may not be a problem. But it is a consideration, especially since I tend to go towards the conservative side with an "over the internet diagnosis" such as this one. I'd certainly do more research if you can, you might get better information than what I'm offering. You are addressing moisture generation within the bathroom. That's good. Other stuff: 1) I do sometimes insulate interior walls, Roxul is good for that. 2) A simple yet effective sound deadener behind your washer and dryer is nothing more than a second layer of drywall over the first. 5/8" is better than 1/2" due to the added mass. Beyond that you can go fancier: 2a) sandwich a 1/2" sheet of homasote between the two layers of drywall. 2b) Use RC clips and metal channel between the studs and the drywall. 2c) Use green glue between the two layers of drywall. 2d) If you do research you'll probably see references to mass loaded vinyl, but I usually use that only when doing fully integrated sound deadening room "systems". I think you'd do better (and save money) with just using drywall.

I just realized that we will be installing a preformed niche or recessed shelf in one of the shower walls. Won't that compromise the vapor barrier? Is there a way to handle that. I did install the poly in horizontal strips but I'm not sure I can easily place another strip behind the shelf and won't that inhibit water from traveling down as it will run into the shelf? What about screwing grab bars, etc, into the tile, won't those also compromise the barrier? I was kind of wondering how the staples and hardie backer screws would affect the barrier too since they puncture the plastic as well, we had a hard time keeping the staples from tearing when it was a big sheet, that's why we went to strips and overlapped them. After reading some more posts, it sounds like I do not need to install a water proofing barrier since I have the vapor barrier in behind the board. It also sounds like I need to use a penetrating grout sealer, not a topical on walls and floors and that Aqua Mix will work AND is low VOC. So now my questions are: 1) how to insert a wall shelf with vapor barrier and hardie already in 2) can items be screwed through the wall into the barrier without causing problems? 3) is it ok to use a steam mop on the sealed caulk floors? 4) is Aqua Mix Seal and Finish the same thing as the Gold version? 5) is the Aqua stuff for sealing both wall and floor grout on both ceramic and slate floors? 6) do I need ANYTHING else to seal or finish the walls or floors? Thank you so much!!!! Now I have to go paint and install wood floors or we will never be done with this. kj

put up the poly film as the vapor barrier. seal it to the studs and bottom plate with a bead of caulk. Use unfaced fiberglass insulation in the stud cavities. You do not want 2 vapor barriers in the same wall.