Moisture-Resistant Backing Material at Walls Behind Tubs and Showers

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Description

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 spaces called wet areas (such as bathrooms), where water is in abundance.

One strategy for managing moisture in wet areas is to use an appropriate waterproof or moisture-resistant backing material at the walls behind bathtub and shower enclosures. These materials prevent moisture from seeping into the wall structure where it could take hold and cause damage. As a Building America best practice, paper-faced gypsum board, most commonly used throughout dry areas of the home, will not prevent moisture penetration and should not be used in wet areas (BSC 2009a).

To prevent moisture issues behind tub and shower surrounds, follow this overall guidance:

  1. Install cement board or an equivalent material behind tub and shower enclosures.
  2. When installing a monolithic tub and shower enclosure (e.g., fiberglass with no seams), know that cement board or an equivalent is not required, but all paper-faced backerboard must meet ASTM mold-resistant standards.

For item 1 above, materials equivalent to cement board are those that prevent moisture penetration, such as fiber cement board, paperless gypsum board, or cement plaster (BSC 2009a). Also, cement board is not waterproof (unless specifically manufactured as waterproof cement board). Therefore, you must coat it with a fluid-applied waterproofing or apply a water resistant barrier behind it that drains (BSC 2009b).

For item 2 above, the ASTM mold-resistant standards are ASTM D3273 or ASTM D6329. If you are using paper-faced gypsum board behind a monolithic surround, check with the manufacturer’s specifications to ensure that the product meets these standards. Also, if gypsum board is used, ensure it is up off the floors by ½ inch everywhere at baseboard locations, which reduces the chances of floor water spills being sucked up into the wall (BSC 2009a).

An important note: When installing the shower and tub, it is essential to air seal and insulate the space 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.

Installing Cement Board Behind Tub and Shower Enclosures

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.  As mentioned previously, cement board is not waterproof; you must coat it with a fluid-applied waterproofing or apply a water resistive barrier behind it that drains (BSC 2009b).

To install cement board:

  1. 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 be easy to install.
  2. Make sure the area behind the tub and enclosure is air sealed and insulated perfectly before installing cement board or equivalent. See Walls Behind Showers and Tubs for air sealing and insulating guidance.
  3. Measure the area for the tub or shower surround.
  4. 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 fitted with a carbide tip. Consider trimming before applying a fluid-applied waterproofing to ensure the board edges remain waterproof.
  5. Attach the cement board to the frame using fasteners specified by code or the manufacturer.
  6. Tape and mud any seams per manufacturer’s specifications or building codes.

Cement Board Installed to a Tub Surround. This image shows the correct installation of cement board behind a tub enclosure

Figure 1 - Cement Board Installed to a Tub Surround. This image shows the correct installation of cement board behind a tub enclosure.  Reference

Cement Board Installed to a Shower Surround. This image shows the correct installation of cement board behind a shower enclosure

Figure 2 - Cement Board Installed to a Shower Surround. This image shows the correct installation of cement board behind a shower enclosure.  Reference

Installing Backerboard behind a Monolithic Tub and Shower Enclosure

If you are installing a monolithic tub or shower enclosure, you have a bit more flexibility in the types of backerboard materials you can select. Because the monolithic surround is one continuous piece, there is less chance of water getting behind it. Therefore, cement board or an equivalent is not required (although it is a Building America best practice). If using paper-faced backerboard, it may only be installed behind monolithic enclosures if it meets the mold-resistant standards ASTM D3273 or ASTM D6329.

The steps for installing paper-backed backerboard behind a shower surround are as follows:

  1. Make sure the area behind the tub and enclosure is air sealed and insulated. See Walls Behind Showers and Tubs for air sealing and insulating guidance.
  2. Check to make sure the wall frames are plumb. This will ensure that the backerboard is plumb and that the ceramic tile or other surface finish will be easy to install.
  3. Measure the area for the tub or shower surround.
  4. Trim the backerboard to fit the measured space. For paper-faced backerboard, ensure it is up off the floors by ½ inch everywhere at baseboard locations, which reduces the chances of floor water spills being sucked up into the wall (BSC 2009a).
  5. Attach the backerboard to the frame using fasteners specified by code or the manufacturer.
  6. Tape and mud any seams per manufacturer’s specifications or building codes.

Ensuring Success

When installing the shower and tub, it is essential to air seal and insulate the space 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.

Carefully measuring and installing the moisture-resistant backboards will help make sure moisture cannot get into the wall. Cutting cement board requires tools different than those used for standard gypsum board, so plan ahead and have abrasive saw blades or carbide-tipped cutters ready. Also be sure that the framing is level and plumb so that the final finish will be smooth and neat.

Scope

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

Water Managed Building Materials

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.

  1. Install cement board or an equivalent material behind tub and shower enclosures.
  2. If a monolithic tub and shower enclosure (e.g., fiberglass with no seams) is used, cement board or an equivalent material is not required, but all paper-faced backerboard must meet ASTM mold-resistant standards.

ENERGY STAR Notes:

In addition to cement board, materials that have been evaluated by ICC-ES according to AC 115, Acceptance Criteria for Waterproof Membranes for Flooring and Shower Lining, 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 according to AC 115, and then only if it meets ASTM mold-resistant standards ASTM D3273 or ASTM D6329.

Training

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Compliance

ENERGY STAR Version 3, (Rev. 07)

Water Management Checklist, Water-Managed Building Materials. 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. In addition to cement board, materials that have been evaluated by ICC-ES according to AC 115, Acceptance Criteria for Waterproof Membranes for Flooring and Shower Lining, 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 according to AC 115, and then only if it meets ASTM mold-resistant standards ASTM D3273 or ASTM D6329.

DOE Challenge Home

Exhibit 1: Mandatory Requirements. Certified under ENERGY STAR Qualified Homes Version 3.

ICC-ES AC 115

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.

ASTM D3273-12

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.

2009 IRC

Section R702.3.8 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 C 1396, C 1178 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 Limitations. Water-resistant gypsum board cannot be used where it will be in direct water exposure or high-humidity areas.*

2012 IRC

Section R702.3.8 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 C 1396, C 1178 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 Limitations. Water-resistant gypsum board cannot be used where it will be in direct water exposure or high-humidity areas.*

 *Due to copyright restrictions, exact code text is not provided.  For specific code text, refer to the applicable code.

More Info.

Case Studies

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References and Resources*

  1. Author(s): BSC
    Organization(s): BSC
    Publication Date: May 2009

    Brochure about creating an air barrier at tub, shower and fireplace walls.

  2. Author(s): DOE
    Organization(s): DOE
    Publication Date: June 2013

    Standard requirements for DOE's Challenge Home national program certification.

  3. Author(s): EPA
    Organization(s): EPA
    Publication Date: June 2013

    Standard document containing the rater checklists and national program requirements for ENERGY STAR Certified Homes, Version 3 (Rev. 7).

  4. Author(s): BSC
    Organization(s): BSC
    Publication Date: May 2009

    Brochure about indoor water management.

  5. Author(s): Dickson
    Organization(s): IBACOS
    Publication Date: December 2011

    Report covering the fundamental waterproofing strategies for tub and shower assemblies.

  6. Author(s): EPA
    Organization(s): EPA
    Publication Date: February 2011

    Guide describing details that serve as a visual reference for each of the line items in the Water Management System Builder Checklist.

Last Updated: 08/15/2013

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