You still need a vapor barrier with cement board, although in this case the barrier can go behind the board. Cement board is heavy enough and breathable enough to take the moisture without a problem. By having the vapor barrier behind the cement board you stop the moisture going further into the wall. When building a steam shower always use a surface membrane. Per Tile Elfs comments, the USG (Manufacturers of Durcok) stated to me that I don;t need a vapor barrier behind their product. This is especially weird since they DO acknowledge that Durock is not waterproof, but rather water-resistant. “A waterproof membrane or vapor retarder membrane must be specified to prevent moisture intrusion and protect adjacent building materials. ” ~B412 Bathtub or shower with prefabricated receptor. Cement board or Fiber-cement board. Consult local building code for moisture barrier requirements. You don't need any cement board under the tub. It is far more important to have a sturdy layer of ply under the tub. Moisture barriers stop that water or vapor from getting any farther and potentially rotting your studs, the drywall or greenboard installed behind the enclosure. They are required beneath your shower pan and behind your shower walls.
You should NOT place a vapor barrier BEHIND the backer board. Where, pray tell, would any such collected vapor/water go? Answer:There's nowhere proper for such moisture to egress. The current best practice is to place waterproofing OVER the backetboard and just UNDER the tile.
Prepping for shower tile is different than the prepping for tile on drywall. A shower is considered a wet application, so you need to waterproof the foundation, which you'll learn how to do in these instructions. After the waterproofing step, the instructions are the same as if you were installing tile on drywall.
Tile Backer Board Breakdown Foam boards are lightweight, easy to cut, and a completely waterproof. Pictured: Wedi Board tile backer board. Schluter approves Kerdi over drywall. Hardibacker is the best known fiber cement board. Cement board has improved over the years and is easier to work with.
Rigid air barrier materials for use behind showers and tubs include fiber-cement, fiber-reinforced gypsum, glass mat gypsum, or fiber mat-reinforced cementitous backer panels.
There are screws made for concrete board. Bugle head like drywall screws but they are coated to resist corrosion and have sufficient strength to support the board. The heads are larger than deck screws or drywall screws, too. Rona, HD, HH will all have them.
The best approach when installing cement board or other tile backer around a tub is to keep the cement board out of the lip area. Apply it to the wall with the bottom edge about 1/4 in. above the lip of the tub. Then apply a generous amount of thin-set to the bottom bathtub tile and position it 1/8 in.
There should be a 1/4" gap between the tub ledge and the backer board. Technically, after the tile is installed over the backer board that gap between it and the tub ledge should be filled with an ASTM C920 sealant (caulking e. g. silicone).
Flooring repairs can be intimidating, but the subfloor under the tub generally consists of plywood secured over floor joists and is fairly simple to replace -- once you get access to it. The entire tub unit requires removal before subfloor replacement can begin.
Concrete backer-board, once commonly referred to in the field by the brand name Wonderboard(r), is a cement product designed as a sheathing material for walls and floors. Most concrete backer-boards on the market today are not water-resistant, but all are waterproof.