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:
- Install cement board or an equivalent material behind tub and shower enclosures.
- 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:
- 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.
- 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.
- 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 fitted with a carbide tip. Consider trimming before applying a fluid-applied waterproofing to ensure the board edges remain waterproof.
- Attach the cement board to the frame using fasteners specified by code or the manufacturer.
- Tape and mud any seams per manufacturer’s specifications or building codes.
Figure 1 - Cement Board Installed to a Tub Surround. This image shows the correct installation of cement board behind a tub enclosure.
Figure 2 - Cement Board Installed to a Shower Surround. This image shows the correct installation of cement board behind a shower enclosure.
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:
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.
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.
Measure the area for the tub or shower surround.
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
Attach the backerboard to the frame using fasteners specified by code or the manufacturer.
Tape and mud any seams per manufacturer’s specifications or building codes.