May, 2008

Building Mold-Resistant Bathrooms

Correct installation and customer education can help prevent moisture problems.

By Steve Easley

A builder from Philadelphia asks, “What is your advice on building and designing mold-resistant bathrooms?”

Bathrooms are certainly more prone to moisture and mold problems than other areas of a home. The primary reasons we see more mold-related problems in bathrooms are:

1. Taking a shower adds half a pound to a pound of water to the air per shower. 
2. Bathrooms usually have showers on exterior walls, which are colder in winter. When warm interior air exfiltrates through the wall and comes in contact with cold exteriors, surface condensation can occur. The higher the humidity in a room, the greater potential there is for condensation to occur in a wall.
3. The gypsum board or drywall used in bathrooms, or even green board, has a paper facing which is a very yummy food source for mold.

Anywhere you have high moisture, you have the potential for mold growth. When the surface relative humidity of a building material gets above 70%, the conditions are ideal for mold to grow and even colonize. (When you see mold, that black or green stuff that grows in tile grout, that means that the mold has colonized. The bad news here is that if you see mold on the surface of tile grout, there is good chance the moisture has penetrated the grout and has been absorbed by the green board behind the tile.)

Many builders mistakenly believe that green board is moisture resistant and therefore an “okay” product to use behind tile around showers and tubs. In my opinion, this is not true. My experience is that green board is nowhere near the best choice to use in bathrooms because, like standard gypsum board, it has a mold-friendly paper face and back. I’ll talk more about solutions to this later. Even though building codes require fans in bathrooms, people rarely use them correctly because they are noisy. Oftentimes the fan and light are controlled by the same switch, so people take a 10-minute shower, and when they leave the bathroom, they turn off the light and fan. That means the fan does not run long enough to exhaust the moisture-laden air from the shower area. The end result is high moisture levels and a perfect environment for mold growth.

Building a mold-resistant bathroom is a multi-step process.

1: Select materials that are not a food source for mold.
Use fiberglass-faced gypsum board such as Georgia Pacific’s Dens Armor Plus on all the walls of the bathroom and on shower and tub walls. I’ve used GP’s Dens Shield Tile backer with great success. You can even use it under tile floors.  The idea here is to install materials with no paper face to support mold growth. Eliminating the food source for mold is job No. 1.

Design the space so that moisture is easily exhausted. I recommend separating the tub/shower and water closet area from the rest of the bathroom. Design the shower/tub and water closet to be in the same room or isolated from the rest of the bath space.  Doing this isolates the shower, the primary moisture source in bathrooms, from the bathroom space. This accomplishes a couple of things: The volume of air to be exhausted is smaller so you remove the moisture faster and more efficiently, thus reducing localized relative humidity. Second, the moisture source, showers and tubs, aren’t spewing moisture throughout the bathroom space, and condensing on expensive bathroom finishes, which dulls them and reduces their attractive service life. (This also results in annoying condensation on mirrors.) 

2: Select and install bathroom exhaust fans correctly.
Make sure the fan you install is big enough for the space and make sure it is ducted to exhaust moisture to daylight—not to the attic!

Control the fan separately from the light. Install a separate timer switch for the light and the exhaust fan so that when people leave the shower, they can turn off the light and leave the fan running until the humid air is exhausted. An alternative is to select a fan that has integral motion sensors and timers that will run it long enough to remove moisture. Some fans have dehumidistats that automatically run the fan until the moisture is removed to an acceptable level.

3: Coach your customers to manage moisture.
• Teach them the warning signs of high moisture, which include condensation on windows, mold growth, excessive condensation on mirrors, and so on.
• Teach them the importance of the proper use of exhaust fans to reduce moisture buildup.
• Make sure exhaust fans vent to daylight.
• Teach customers to watch for any sign of mold.

STEVE EASLEY is president of Steve Easley & Associates, which consults and trains on building science issues. For more info. Visit www.codecollegenetwork.com.