Cement Backer Board
DEAR TIM: We are getting ready to install new ceramic tile in a shower and a bath tub. It appears that there are several materials to use beneath the ceramic tile. Some of the products are specially treated gypsum based drywall. Others are cement boards. Which material is the easiest to work with? Which product is waterproof? Should I be concerned about water vapor if my shower or tub is on an exterior wall? Ursula B., Fremont, NE
DEAR URSULA: The absolute harshest environment inside a home, in my opinion, is a bathroom. Here is the battleground between water and building materials that do not like water. Leaks originating from plumbing fixtures, splashing water, leaks around shower curtains and doors, and vast quantities of water vapor will readily cause regular drywall, wood framing, wood sub-floors, etc. to rot and deteriorate. Ceramic tile and properly mixed grout will not deteriorate when in contact with water. You should absolutely use a tile backer board that is also unaffected by water.
The special gypsum products you refer to are moisture resistant drywall. These products have a water repellent emulsion added to the gypsum core to make them behave better than ordinary drywall in the presence of water. Even the paper that surrounds the gypsum core has water repellents in it. Some other gypsum products have special glass mats and polymer coatings that resist water and water vapor. However, there is a key word missing in all of the product descriptions. That word is waterproof. The specially treated gypsum products will not last forever if they are allowed to get wet.
If you want a backer board for your ceramic tile that will not deteriorate when water gets through your grout, then you better look to the cement board products. Properly mixed concrete or cement board is not harmed by the presence of water. That is why concrete is an excellent outdoor building material. Years ago the tile setters of old mixed and placed wet concrete on the walls of old houses. The ceramic tile was actually cemented to this waterproof substrate. That is why you rarely see a ceramic tile failure in an old house. If you do see a failure, it is often traced to a leak that rotted the wood that was holding up the cement and tile.
There is no doubt that the gypsum based products are the easiest to work with. You cut these with a simple razor knife. However, you pay a price for this convenience. The price may be a complete new tile and backer board installation job within 7 - 10 years. The cement backer boards are heavier and are harder to cut. They also are slightly more expensive. The extra work and cost are an investment. If you follow the instructions provided by the manufacturers and the Tile Council of North America, you very likely will have a permanent ceramic tile installation.
Your vapor barrier concern is very valid. Showers and tub baths produce massive amounts of water vapor. This vapor is concentrated in the bath area. It would be a great idea to incorporate a seamless vapor barrier on the walls of your bathroom if at all possible. This vapor barrier should be applied to the walls after the insulation is in place but before any finish wall materials are attached. Cross laminated high density polyethylene plastics offer the best protection. These materials are slightly more expensive that the regular 4 and 6 mil clear poly vapor barriers most builders use. The high quality vapor barriers can often be found at supply houses that sell concrete supplies to contractors.
Pay particular attention to the installation instructions when installing your tile backer boards. If you decide to go the convenience route, you must not let the gypsum products touch the tub or shower pan. You need to leave a one quarter inch gap. If these products touch the plumbing fixture lip, they can readily wick water into the gypsum core and paper. This will accelerate their eventual deterioration.