You must coat it with a fluid-applied waterproofing or apply a water-resistant barrier behind it that drains ( BSC 2009b). Cement board comes in the same standard sheet sizes as regular gypsum board: both are 5 feet wide, which is also the width of a standard tub surround. Unless specifically manufactured as a waterproof product, cement board is water resistant but not waterproof. Equivalent materials include coated glass mat which has a gypsum core with glass fibers sandwiched between fiberglass surface mats and fiber cement which is reinforced with wood fibers. Appropriate materials include cement board which has a cementitious core and glass mats on both sides to strengthen the board.

Use an appropriate waterproof or moisture-resistant backing material behind grouted tile or stone or segmented plastic or fiberglass tub and shower surrounds. Special protection is needed behind showers and tubs to prevent moisture from seeping into the wall structure. Complicating matters are wet areas where water is intentionally brought into the home, such as bathrooms and kitchens. As with water seeping in from outside, unmanaged water inside the home can lead to a host of problems including mold, mildew, air quality issues, and even structural damage. However, interior moisture management is just as important, with many of the same consequences if it fails. Moisture management is often thought of in terms of keeping exterior moisture outside of the building envelope.

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