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Building Science

Adjusting Bath Fan Use in Winter

Do you really need to run it when you shower?

Condensation on a bathroom mirror after a morning shower is a common occurrence. But is the presence of condensation worrisome enough to warrant using the bath fan?
Image Credit: Energy Vanguard

You may have heard or read somewhere that you should run your bathroom exhaust fan whenever you take a shower and then let it run for a while after you’re done with the shower. Showers increase the humidity in the bathroom. Sometimes it gets high enough to cause condensation to appear on the mirror and other surfaces in the bathroom. And that can result in mold growth.

So you should always run your bath fan when you shower. Or so they say.

Reasons to run the bathroom exhaust fan

Bath fans are a really nice thing to have. I know. I lived without one in my 48-year-old condo for years before finally remodeling and installing an exhaust fan two years ago. They do indeed remove moisture. I took the photo above one day last summer after intentionally leaving the fan off so I could get a good picture of condensation. (Yeah, I’m like that.)

But they also remove odors. Bathrooms have been known to be smelly on occasion. You know, with all the candles and incense and hairspray and stuff. Right? Oh, and then there’s the cleaning products. And, in some bathrooms (not mine!), a pile of dirty laundry. Seems like there’s something else, too, but anyway, you get the point. Bath fans are good for removing odors.

Another reason to run the bath fan is if it’s part of your whole-house ventilation system. You can get controls to run them continuously or a certain number of minutes per hour. Some fans have the controls built in. Either way, if the bath fan is part of your whole-house ventilation, you don’t want to turn it off in winter. (By the way, bath fans don’t have to be part of exhaust-only whole-house ventilation. You can pair them…

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2 Comments

  1. user-6890838 | | #1

    Dangerous Ground
    Allison, Given the difficulty of controlling where air leaks out of a building and the ease of controlling the moisture content of the air, it seems the benefits (to the house and the breathers who live there) of drying things out as quickly as possible far outweigh the energy loss associated with bath fan exhaust.
    The same leaks that make indoor air dry when it's cold outside will invite wet bathroom air to find a condensing surface somewhere we can't see or clean up. Luckily, sealing up the home may minimize overly-dry air in the winter AND keep indoor air from condensing in the walls or attic. However, I don't think retiring the bath fan is justified with or without the air sealing: too much risk of indoor high moisture levels leading to condensation somewhere.
    Maybe stuff dries out relatively quickly in Florida. After the cold snap passes, condensation-wetted sheathing may have a chance to recover. But there are too many parts of the US where the "cold sanp" lasts four or five months. And in those locations failing to exhaust bathroom moisture promptly is a sure way to tip the conditions toward mold-favoring.
    So, if what you're saying is it's ok to not use the bath fan in areas where the average winter temperature is 50 deg or higher, and the outdoor temperature will be 30 deg or lower for between 20 and 60 hours, and there is enough leakage in the house to have dry air but not enough leakage to allow exfiltration-induced condensation, then yeah. But for the rest of us, we're fans of bath fans.

  2. User avater
    Reid Baldwin | | #2

    Magnitude of the effect
    I agree that not running the bath fan during a shower is directionally correct when the indoor humidity is lower than desired. Have you done any calculations to evaluate whether or not the magnitude of the effect is significant? If the magnitude of the benefit is tiny, then we want to stick to very simple guidance that will always protect against mold.

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