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Preventing mold with bathroom ventilation

by BrianReeves
mold free bathroom

A properly installed vent fan is an essential weapon against bathroom mold.


Most people understand that mold likes moisture. That’s why bathrooms are so vulnerable to mold. Water that gets onto the bathroom floor after showering or bathing is an obvious concern. But waterproof flooring materials like ceramic tile or sheet vinyl do a good job of minimizing the mold potential of water that gets on the floor. Of greater concern is moisture in the air –moisture that can permeate wall and ceiling materials because of poor bathroom ventilation.

When the bathroom fills with a fog of moist air while someone is taking a hot shower, thousands of moisture droplets can condense on cool wall, ceiling and window surfaces in the bathroom and also in adjacent rooms. Some of this moist air can even penetrate into unseen building cavities through cracks and gaps around electrical outlets and molding. Because damp organic material (wood, paper-faced wallboard, paint resins, paper-faced fiberglass insulation) makes ideal mold food, we have the makings of a mold invasion.


Ventilation to the rescue –sometimes

The bathroom vent fan is a major weapon against bathroom mold. The fan’s job is to move moist air outside the house before it can condense and permeate into mold-prone materials. Most building codes require that bathrooms be equipped with “active ventilation” in the form of a ventilation fan. However, the bath fan only helps to prevent mold and moisture damage if it’s turned on during bathing activities and kept on until moist air is moved outside. Who can say whether or not the next person to take a shower will remember to turn on the fan?

Another problem that can occur with bathroom vent fans has to do with how they are installed. Some builders mistakenly allow the fan to blow moist air into the attic, a practice that simply moves the mold problem to another part of the house. During cold weather, warm, moist air blown into a cooler attic will deposit its moisture on attic rafters and roof sheathing. Telltale black mold stains typically result from this ventilation error. Eventually, this mold can develop into wood rot.


Bathroom ventilation done right


Fortunately, there are several ways to make sure the bath fan is used properly to evacuate moist air. First and foremost, the fan must be ducted to the exterior, not to the attic. Best practice for bath fan installation also includes using spray foam or caulk to seal air leaks around the ceiling opening for the fan, and covering the fan with attic insulation.

To ensure that the fan is always used to exhaust moist air from the bathroom, the fan can simply be controlled by the same switch that operates to bathroom’s main light fixture. The fan comes on with the light, whether it’s needed or not. Another approach favored by many homeowners is to install a fan that is controlled by a humidistat. This device measures the bathroom’s humidity level, turning on the fan when a preset humidity level is detected. This type of sophisticated control will also turn the fan off automatically when the humidity falls below a preset level. Manufacturers like Braun and Panasonic are leading the way in developing advanced controls for bath fans that are also quiet and energy efficient.

Interested in learning about mold and how to get rid of it? I recommend checking out the EPA’s guide for mold cleanup and this very informative mold learning center.

Tim Snyder is an author specializing in home building topics such as mold and energy efficiency. 

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vanessawilson April 5, 2014 - 3:25 am

This very interesting information about bathroom ventilation thanks for sharing.

Tommy February 5, 2015 - 8:17 am

Thanks for sharing, we had a slight mold issue I was able to get a handle on before it got to bad. My kids are horrible with turning the fan on when showering and I am convinced this was the ultimate issue, well that and the fan was not working 100% of the time. Mold removal can be such a costly en-devour!
When shopping for a new Bathroom exhaust fan I found the following website helpful.

Punitha June 10, 2015 - 3:49 am


Thanks for this informative blog. I’m considering have a window to allow a better air circulation in the bathroom. Would that a good option to avoid mold?

Bappy June 25, 2015 - 9:34 pm

Hey there, I really appreciate your post, it was really informative. I’ll be looking forward for Home, or kitchen exhaust fan your next post….

Annette J. Gordon March 8, 2016 - 3:37 am

Great post. Which manufacturer/model would you recommend the most for ventilation? At the moment I’m looking to get Panasonic 110 CFM for my bathroom…

abd June 27, 2016 - 9:56 am

Yes indeed, a window is better than nothing..

Selena July 7, 2016 - 2:54 am

Treating with mold is a difficult task but the more annoying task is to recognize the mold and its symptoms. Most of times homeowners take it lightly but their after sideffects are too dangerous. Types of molds and its symptoms http://www.floodaz.com/types-of-mold/

Puspendu Seth October 25, 2016 - 4:15 am

This is a nice post about exhaust fan. Exhaust fan in hails fresh air from outside and remove air from inside the room. This post has shown some great model of exhaust fan. So thanks for writing such a awesome post.

Homer Parker December 28, 2016 - 2:13 am

Mold along with the bathroom ventilator is costly but this didn’t exist for a long time for me. my kids used to play with these exhaust fans as the wall mounted is little low in my home. I finally checked for various sources and finally got the exact exhaust and the ventilation fan for my bathroom from http://bestfanreviews.com/

Bernice Reid December 8, 2017 - 10:46 am

My family and I live in Michigan. We live in an apartment on the top floor. When it is very windy outside, dark flakes come out of the bathroom ceiling fan vent. This has happened more than once. I think this started after some roofers did roof work (about two years ago). Last week, a lot of dark flakes came out of the bathroom vent. The flakes got on the toilet and floor. I called the housing inspector days ago, but he has not called me back. A maintenance guy came over yesterday, and he vacuumed out the vent. Do you know what the dark flakes are? Can they cause health problems? The maintenance guy said the stuff is dirt. The flakes don’t look like dirt.

NFPA Construction Group September 6, 2019 - 11:07 pm

Thanks for sharing this awesome blog with us.


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