Install cement board or an equivalent moisture-resistant backing material on walls behind tub and shower enclosures composed of tile or panel assemblies with caulked joints.
- Don’t use paper-faced backer board, i.e., paper-faced drywall, behind seamed tub and shower enclosures.
- Use an alternate approved product such as fiber-cement, fiber-reinforced gypsum, glass mat gypsum, or fiber mat-reinforced cementitious backer panels.
- If a monolithic tub and shower enclosure (e.g., fiberglass with no seams) is used, a paper-faced backer board can be use if it meets ASTM mold-resistant standards (ENERGY STAR).
See the Compliance Tab for related codes and standards requirements, and criteria to meet national programs such as DOE’s Zero Energy Ready Home program, ENERGY STAR Single-Family New Homes, and Indoor airPLUS.
Moisture management is often thought of in terms of keeping exterior moisture outside of the building envelope. However, interior moisture management is just as important, with many of the same consequences if it fails. 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. Complicating matters are wet areas where water is intentionally brought into the home, such as bathrooms and kitchens.
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 (BSC 2009b). Paper-faced gypsum board will not prevent moisture penetration and should not be used in wet areas (BSC 2009a). Non-paper-faced fiber-reinforced gypsum board, moisture-resistant gypsum board, or “green board” products are also not recommended behind tub surrounds (BSC 2009a).
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.
How to Install Cement Board Behind Tub and Shower Enclosures
- Check to make sure the wall frames are plumb. This will ensure that the cement board is plumb and that the ceramic tile or other surface finish will install correctly.
- 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. See Walls Behind Showers and Tubs for air sealing and insulating guidance.
- Measure the area for the tub or shower surround.
- Trim the cement board to fit the measured space. Use a circular saw fitted with an abrasive blade or a hand tool specific to the job and fitted with a carbide tip. Apply a fluid-applied waterproofing to the board including the edges.
- If the tub or shower is on an exterior wall and the cement board will serve as an air barrier over the insulation, apply a thick bead of caulk to the surface of the exposed studs, wood blocking, and tom and bottom plates. Nail or screw the thin-profile air barrier material to the studs.
- Tape and mud any seams per manufacturer’s specifications.
When installing a shower or tub on an exterior wall, it is essential to air seal and insulate the wall cavities behind the shower and tub enclosure to prevent thermal bypasses that can result in moisture, heating, and cooling problems. The Solution Center Guide Walls Behind Showers and Tubs provides information about how to air seal and insulate behind a shower and tub.
Cutting cement board requires tools different than those used for standard gypsum board; use abrasive saw blades or carbide-tipped cutters. Be sure that the framing is level and plumb before installing backer board to avoid alignment issues with premanufactured tub surrounds or tile.
No climate-specific information applies.
The Compliance tab contains both program and code information. Code language is excerpted and summarized below. For exact code language, refer to the applicable code, which may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.
Water Management System Builder Requirements
4. Water-Managed Building Materials.
4.2 Cement board or equivalent moisture-resistant backing material installed on all walls behind tub and shower enclosures composed of tile or panel assemblies with caulked joints. Paper-faced backerboard shall not be used.17
Footnote 17) In addition to cement board, materials that have been evaluated by ICC-ES per AC 115 may also be used to meet this requirement. Monolithic tub and shower enclosures (e.g., fiberglass with no seams) are exempt from this backing material requirement unless required by the manufacturer. Paper-faced backerboard may only be used behind monolithic enclosures or waterproof membranes that have been evaluated by ICC-ES per AC 115, and then only if it meets ASTM mold-resistant standards ASTM D3273 or ASTM D6329.
Please see the ENERGY STAR Single-Family New Homes Implementation Timeline for the program version and revision currently applicable in in your state.
Exhibit 1 Mandatory Requirements.
Exhibit 1, Item 1) Certified under the ENERGY STAR Qualified Homes Program or the ENERGY STAR Multifamily New Construction Program.
Section R702.3.8 (R702.3.7 in 2015 and 2018 IRC) Water-resistant gypsum backing board. Gypsum board used as a base or backer for adhesive application of ceramic tile or other required nonabsorbent finish material must conform to ASTM C1396, C1178, or C1278 . Use of water-resistant gypsum backing board is allowed on ceilings where framing spacing is 12 inches or less on center for ½-inch thick or 16 inches for 5/8-inch thick gypsum board. Water-resistant gypsum board cannot be installed over a Class I or II vapor retarder in a shower or tub compartment. Cut or exposed edges must sealed per manufacturer’s recommendations.
Section R702.3.8.1 (R702.3.7.1 in 2015, 2018, and 2021 IRC) Limitations. Water-resistant gypsum board cannot be used where it will be in direct water exposure or high-humidity areas.
Section R102.7.1 Additions, alterations, or repairs. Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with the requirements of this code, unless otherwise stated. (See code for additional requirements and exceptions.)
Appendix J regulates the repair, renovation, alteration, and reconstruction of existing buildings and is intended to encourage their continued safe use.
Acceptance Criteria for Waterproof Membranes for Flooring and Shower Lining. Available for download from ICC Evaluation Services. The criteria applies to liquid-applied and sheet membranes used as barriers to positive liquid water migration in load-bearing, bonded, thin-set installations of ceramic tile and dimension stone on floors, and as shower sub-pan lining.
Standard Test Method for Resistance to Growth of Mold on the Surface of Interior Coatings in an Environmental Chamber. Available from ASTM. The standard test method to determine the resistance of interior coatings to mold growth.
ASTM D6329-98 (2008)
Standard Guide for Developing Methodology for Evaluating the Ability of Indoor Materials to Support Microbial Growth Using Static Environmental Chambers. Available from ASTM. The guide describes an approach to evaluate the ability of a variety of material to support microbial growth using a small chamber method.
Installing new tubs and showers and replacing the tub and shower surround are popular bathroom remodeling projects. When conducting these projects, install a new layer of cement board or an equivalent moisture-resistant backing material behind the tub or shower, as described in the Scope and Description tabs of this guide.
See Compliance tab.
Access to some references may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.