In addition, because showers and bathtubs are highly moist areas, they need different kinds of substrate than other tile applications. Tile on a relatively dry kitchen floor can be laid on plywood; tile on a shower surround cannot use bare plywood as substrate.
The common element in these shower-safe applications is what is called a cementious backer unit or CBU. Brand names Durock, Denshield, Hardiebacker, and Wonderboard are all CBUs. They are made of 100% inorganic materials that will not rot, shrink, or decompose.
Acceptable backer boards for showers and bathtub surrounds:
Cement Board + Plastic Sheet
A moisture barrier of 4 or 6 mil plastic is installed on the studs. Then, the cement board is installed. Tile is laid onto the cement board.
Cement Board + Liquid Membrane
Cement board is installed directly on the studs. No plastic sheeting goes behind the cement board. A liquid membrane, such as Redgard or Hydroban, is applied to the cement board. After drying, tile is installed.
Cement Board + Sheet Membrane
Cement board is installed directly on the studs with no plastic sheeting behind it. Sheet membrane such as Schluter Kerdi is applied to the cement board with thinset.
After the thinset has dried, tile is applied to the sheet membrane with thinset.
Membrane-Faced Board Only
Backer board is available which integrates the properties of the cement board and the moisture barrier. Georgia-Pacific's Denshield is one such product. An acrylic coating forms a sealed moisture barrier on top of a glass mat backer board.
Backer Board on Drywall Instead of Directly on Studs
All of the previous acceptable options are installed directly on the open studs. But if you happen to already have drywall in place, you can install these tile backers on the drywall. Removing the drywall is not necessary. However, in most cases you will want to do this anyway to access plumbing and to better control the thickness of the installation.
Drywall as the only backer for tile not an option. The chance of moisture infiltration is too high to risk this. It will not take much water for drywall's paper facing to disintegrate and turn moldy. Even a tiny amount of water introduced through a crack or hole in the tile will expand once it hits the moisture-hungry paper facing and gypsum core.
Greenboard is only slightly more water-resistant than plain drywall. It has drywall's same gypsum core and paper facing. However, the facing is impregnated with waxes that shed water better than conventional drywall's paper. Code does allow for greenboard as a tile substrate in showers, but nearly all tile installers recommend against this.
Plywood alone cannot be used as substrate under tile in showers.
Some homeowners believe that painting or priming plywood will render it suitable to use as shower/tub backer board. This is not true.