Even with proper ventilation, shower steam exposes bathrooms to more moisture than any other room in the house. Bathrooms require materials that will not leak or hold moisture that damages underlying framing materials. The materials must also repel water so they do not breed mould and mildew. While the ceiling in a shower stall does not get the same exposure as the floor, it is still exposed to water.
Water Resistant Sheet
Water resistant sheet rock is available, and is usually called "greenboard" because of its green hue. While it may be referred to as waterproof, greenboard is not. The material is moisture-resistant. It is a better alternative to cover bathroom walls and ceilings than regular gypsum or sheet rock because of its water resistance. Like regular sheet rock, it should not be used as a sub-straight for tile.
A vapour barrier is critical element in tiled bathrooms. Even thoroughly grouted tiles are not waterproof. Water (and some tiles) will wick water through to the substrate, so a waterproof membrane, or "vapour barrier," must be installed in between the tiles and whatever substrate or "backerboard" is used on shower stall floors, walls and ceilings. Tile ceilings benefit from vapour barriers because they are particularly prone to moisture from condensation.
Substrates, or "backerboard," for shower and bathroom tiles are not always waterproof. If they have been waterproofed with a vapour barrier, they probably do not need to be. Using waterproof material in addition to using structural backerboard instead of sheetrock gives you extra insurance against structural damage, mould and mildew. Cement board, which is a cement-infused mesh sheet, is a common waterproof substrate. It prevents water damage from condensation when used for tile ceilings in or near shower stalls.
Some tiles are porous and not well-suited for bathroom and shower applications. Glazed ceramic tiles have a number of advantages over porous materials. While water may wick through ceramic, it will not permeate ceramic glaze. The ceramic finish is also easy to clean and is an inhospitable environment for mould, mildew and bacteria. This can be particularly helpful on ceilings where droplets of condensation may actually remain longer than beads of water on a wall; Moisture on the walls tends to run down the wall instead of beading up until they evaporate.