Basic drywall is not approved for any type of installation in a moist area. The paper facing and gypsum core of drywall are both easily invaded by water, and the material will crumple and mold if exposed to high levels of moisture. In terms of alternative materials, there is a difference between water-resistant and waterproof. Water-resistant products can withstand high levels of moisture without absorbing the water from the air. However, these products cannot stand up to direct applications of water, or they too will crumble. Waterproof material can withstand direct applications of water without damage.
Drywall is a basic component in most building and construction projects, but not all drywall is created equal. Regular drywall is resilient until challenged by moisture, and high levels of humidity can actually crumple the material from the inside out. For installations in wet areas of the home such as basements and bathrooms, contractors must turn to one of two alternative materials: green board and cement board.
Water-Resistant Versus Waterproof
Green Board Characteristics
Green-board drywall is water-resistant, not waterproof. It gets its name from the telltale color of the paper facing on the front and back. The core of the green board drywall is a thicker and more resilient version of the drywall's gypsum core, which allows it to withstand indirect moisture applications, such as the humidity in a bathroom or near a laundry room. However, the paper and gypsum will still absorb water if it is applied directly to the surface, and so the applications of green board are still limited by water exposure.
Cement Board Characteristics
Cement board, also known as backer board, is fully waterproof and can withstand direct applications of water. The name backer board comes from its use as a subsurface for tile installation, especially in showers and tub enclosures. The core of these boards is made up of a combination of cement and concrete that will not absorb any water, and therefore will undergo no damage, even if exposed directly to water. For this reason, it works well as a tile support; the moisture from the mortar and grout used to install the tile will naturally wick into the subsurface, but cannot damage cement board.
Because cement board is thicker and heavier than green board, it is harder to install. However, green board and cement board are installed essentially the same way as regular drywall: the panels are attached to floor joists or wall studs with galvanized or corrosion-resistant nails or screws. All panels can be cut with a sharp utility knife, though because cement board is thicker you will need to score both sides of the panel before it will snap in two. For bathroom applications, green board can be used on ceilings and walls where there is no direct water application, such as behind sinks and on bare walls. Only cement board can withstand the wear and tear of a shower or tub enclosure.