Special protection is needed behind showers and tubs to prevent moisture from seeping into the wall structure. Use an appropriate waterproof or moisture-resistant backing material behind grouted tile or stone or segmented plastic or fiberglass tub and shower surrounds. Appropriate materials include cement board which has a cementitious core and glass mats on both sides to strengthen the board. 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. 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. You must coat it with a fluid-applied waterproofing or apply a water-resistant barrier behind it that drains. Paper-faced gypsum board will not prevent moisture penetration and should not be used in wet areas. Non-paper-faced fiber-reinforced gypsum board, moisture-resistant gypsum board, or “green board” products are also not recommended behind tub surrounds.
Selected from: https://basc.pnnl.gov/resource-guides/cement-board-installed-behind-tile-and-panel-tub-and-shower-enclosures#quicktabs-guides=1
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