If the tub or shower is on an exterior wall, make sure that any piping or wiring penetrations through the walls behind the tub and enclosure are air sealed and that the wall cavities are insulated before installing cement board or equivalent.This will ensure that the cement board is plumb and that the ceramic tile or other surface finish will install correctly. Check to make sure the wall frames are plumb.How to Install Cement Board Behind Tub and Shower Enclosures

When installing a shower or tub on an exterior wall, it is essential to air seal and insulate the wall cavity behind the shower and tub enclosure to prevent thermal bypasses that can result in moisture getting into the walls and cold tubs. The Building America Solution Center guide Walls Behind Showers and Tubs provides information on how to air seal and insulate behind a shower and tub. Non-paper-faced fiber-reinforced gypsum board, moisture-resistant gypsum board, or “green board” products are also not recommended behind tub surrounds ( BSC 2009a). Paper-faced gypsum board will not prevent moisture penetration and should not be used in wet areas ( BSC 2009a). 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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