How do I prevent the wallboard behind the bathroom tiles from swelling up?
When water repeatedly gets into ordinary gypsum board, it softens and disintegrates, causing ceramic tiles to bulge or fall off. The key defense is to prevent the water from getting in. That means use a good grout and maintain it with regrouting when it cracks and silicone sealer every couple of years. The caulking between the tub and the tile must be continuous and remain flexible or water will get in there also. Use flexible caulk as well in the corner where the two walls come together to allow for building or panel movement. Install the wallboard 6 mm (1/4 in.) above the tub so that minor leaks won't have a chance to soak into the gyps. Using water-resistant panels gives you a greater margin for error and using special cement-board panels will avoid all future problems.