GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Best Practices in Mold-Resistant Bathrooms

user-1086760 | Posted in Green Building Techniques on

GBA Community,
I have looked at a variety of best practices in mold-resistant bathroom strategies and materials for a moderate size full bathroom. I have landed on the following:

For materials, a project must use a non-paper faced wall covering such as a gypsum board faced with a fiberglass laminate/coating for the entire bathroom (Densarmor is one product example). A mold-resistant, paper-faced product like Greenboard would not suffice. Concrete board should be used to back behind the tub and floor if tile is being used. A full piece fiberblass tub/shower does not need to be backed with concrete board although it will need an air barrier with insulation behind it if it is on an exterior wall (I think an air barrier should be required for interior walls as well).

For a ventilation, the project should meet the requirements of ASHRAE 62.2-2010 and require 50 cfm of intermittent bathroom exhaust to the outdoors (or 20 cfm of continuous exhaust). The fan should be tied to the main light switch and the light switch or fan should be equipped with a delay off timer set to at least 15 minutes. This will allow for the fan to continue running once the light has been switched off and the resident has left the bathroom.

Now I know there are a thousand ways to skin this cat, but I would really love some feedback on best practices that are not only effective, but low-cost and applicable in the field. Please specify the climate zone(s) you are thinking about when suggesting a best practice. San Diego is not New Orleans or New York City and vice versa. Also, if you have recommendations, please suggest ones that don’t rely on the resident’s good behavior.



GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    Most moldy bathrooms get that way because of occupant behavior, not the choice of building materials. If people don't wipe up puddles of water on the floor, and never use the exhaust fan, and keep the relative humidity of their entire house at a high level by installing a humidifier, you'll get mold.

    If your family is reasonably clean and responsible, and you don't install a humidifier, and you wipe up your puddles and use the fan, you'll be fine.

  2. user-1086760 | | #2

    Hi Martin,
    I completely agree that resident behavior is critical to keeping mold out of a bathroom, as well as ensuring the successful of operation of any part of green building, however, just because you teach someone how to properly turn off a light when not using a room or leaving the house, doesn't necessarily mean that you install an incandescent over an LED. Or just because you teach someone that they can keep their thermostat at 65 if they wear a sweatershirt and slippers in the home as opposed to their tank top doesn't mean you put in electric resistant baseboard heat either.

    So, let's say that are writing this spec for a city or state program and you wanted to make sure you did everything possible to make sure it stayed mold-free, but left the resident as a variable you couldn't control due to the size of the program, and your inability to personally go out and visit each resident and instruct them on the best practices of keeping a bathroom mold-free. What precautions in materials and methods that you would take? Where is the cut-off in terms of cost-benefit?

    I understand there is not a definitive answer here, because the resident will always be a variable to a degree. I just want to know what the great minds out there think is a reasonable strategy to ensure a relatively mold-proof bathroom.


  3. user-1075855 | | #3

    I would add that you might want to add in a waterproof membrane behind your tiles.

    If you can, try getting one of those fans that can sense humidity and turn off and on based on that.

  4. user-716970 | | #4

    Most cheap bath fans get ignored or disconnected due to their unacceptable noise levels. Why not spec a fan such as the Panasonic Whispersense, which features motion sensor and dehumistat controls, for totally automatic use. Much of the mold risk would be eliminated.

  5. user-1086760 | | #5

    Garth, I agree Panasonic makes a great fan. I have heard stories and seen bathrooms where the fan has been disconnected because of the noise, so this a real issue.

    I don't have any experience with motion sensor controls on fans, but I have heard second hand that humidstats tend to have a short life in a bathroom. Supposedly the sensor in the humidstat, a wick, becomes dirty after awhile and starts to malfunction making the fan work overtime. After it starts to malfunction, it is rarely replaced by the homeowner or maintenance staff.

    As for the waterproof membrane behind the tiles, do you have any recommendations Jay? I usually just turn to concrete board.

  6. Billy | | #6

    Use a Kerdi membrane over the backboard and set the tile on the Kerdi.

    Use a fan/light control that turns on the fan when you turn on the light and leaves the fan on for up to 20 minutes (adjustable) after you turn off the light.


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |