People ask “what is Green board” and “what do you use it for?’. Green board is a type of drywall that is commonly used in bathrooms. It is moisture and mold resistant, but not waterproof.
Green board is used mostly for damp environments, but not to be confused with wet environments. You do not want to use green board on the tub/shower walls because if the grout or caulk should fail, the paper that lines the green board is only moisture resistant. Should the water get to the gypsum part of the wall, this will eventually lead to water damage, and tiles potentially falling off the wall. Additionally, green board is wrapped in paper, which is an organic product. Organic products produce mold when they come into contact with water and are not allowed to dry out thoroughly.
Structural damage could even potentially occur if not corrected in time. So, again green board is generally used for the walls in the bathroom that will not come in direct contact to water.
Cement board is sometimes called Durock, Wonderboard, and Hardibacker. It is used for wet applications in tubs walls, shower walls, and floors. It’s a cement based product with a mesh glass underlayment in it which makes it waterproof and very durable. Cement board is normally found in ¼” and ½” thicknesses and is available in 3 x 4, 3 x 5, and 3 x 6 sheets, depending on the manufacture. Should the grout or caulk fail, the wall is waterproof, and therefore cannot rot and since there is no paper, there is no mold hazard either.
So, remember green board for damp areas but cement board for all areas that could possible come in direct contact with water to ensure the longest life of your bath and tile needs.