Waterproofing Tile Installations
Standard bathtubs have a lip on three sides of the tub to capture water. The fourth side is of course the long side that you pass over getting into and out of the tub. Porcelain coated steel tubs have a much higher lip than cast iron tubs. Cast iron tubs have a very slight rise that will capture and divert water back into the tub, however, you must make sure the tub is installed level in both directions.
Have you ever noticed that water can travel up a paper towel or rag that you suspend into a bowl of water? This is caused by capillary attraction. The same thing can happen with cement board or water resistant drywall that is allowed to 'sit' in a puddle of water along a tub edge.
When you install ceramic tile board around a tub you do NOT want it to contact the surface of the tub. I recommend a 1/4 to 3/8 inch space between the bottom of the board and the tub or shower surface. You can caulk this crack if you like with pure silicone caulk.
Tar Paper or Vapor Barriers
What happens if water somehow gets past your cement backer board? You can stop it cold in its tracks and divert it back to the tub or shower basin if you install tar paper or a plastic vapor barrier on the wall studs before you install your ceramic tile backer board. You must make sure that the vapor barrier or tar paper laps into the tub. If you use tar paper make sure that different layers overlap one another like roofing shingles. You don't want water running behind a sheet of tar paper. Don't trim off the excess material until after you have installed the ceramic backer board. Then carefully cut it back so that it is flush with the outer surface of the ceramic backer board. When the tile is installed, the vapor barrier or tar paper will be recessed 1/4 inch from the finished edge.
Caulk ALL Seams BEFORE Tilework
As you install each sheet of ceramic tile backer board, leave a 3/16 or 1/4 inch space between sheets. These cracks will be caulked with 100 percent pure silicone caulk BEFORE you begin to install ceramic tile. I have had great success doing this. If you then decide to tape the joints with thinset and fiberglass tape, you can do so AFTER you have caulked.
If you do tape with thinset, you must be careful NOT to create a hump at the taped seam. Ceramic tile is very unforgiving. The tile will not sit evenly as it crosses the humped area. Add to this the high gloss surface of most ceramic tile and you have a recipe for disaster. The high gloss surface will visually transmit the defect.
Grout is NOT Waterproof
Contrary to popular belief, grout is not waterproof. Water can travel through grout and especially past the contact point between the grout and the ceramic tile.
The joint between the tub and shower must never be filled with grout. This joint should be filled with silicone caulk only. I always grout the tub or shower areas and dig the grout out of this crack while it is still soft. It is very hard to try not to grout it. Caulking the crack before grouting is a huge mistake.
After grout has cured for approximately one week it is a great idea to seal it with a silicone sealant or better yet a silane-siloxane water repellent. Silicone is a film forming sealant that cannot breathe. You can get a great silane-siloxane water repellent from Saver Systems.