Q. In winter, I’m wondering if it would help bring moisture to my house and reduce leakage of warm air if I don’t use the bathroom exhaust fan while taking a shower. I don’t want mold to grow in the shower, but I know all that warm air is going right out the ceiling.

Barb H.
Milwaukee, Wis.

A. Dearest Barb,

Have you considered leaving the fan off for good and redecorating your bathroom with a jungle theme? You know, green walls, potted bromeliads, monkey shower curtain? That way, the occasional clouds of steam will just add to the ambience.

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Of course you haven’t, Barb, because that would be a highly inefficient way to deal with moisture buildup in the bath. But you’re right that one of the standard solutions to the fogged-up mirror issue – running the bathroom exhaust fan – is problematic in the cold months (and the hot months too, if you have AC). The fan’s job, after all, is to shoot your humid, heated air out of the house. That household air needs to be replaced from somewhere, and when that somewhere is the frigid Wisconsin winterscape outside, your heating system has to kick in to get the new air supply up to snuff.

Running the heat more than necessary: Boo! But nobody likes a dripping bathroom, either, for mold as well as laundry reasons. Here’s a deceptively simple solution: Leave the door open when you shower. The steam will dissipate more evenly into the house, keeping the bathroom drier while increasing humidity elsewhere. And that, you may recall, is an energy win because the more humid the air, the lower you can set your thermostat while still feeling comfortable. Note: This approach requires cooperation from any housemates you may have, and perhaps an opaque shower curtain (those monkeys are looking better and better).

If this is a nonstarter, you can still deal with the steam buildup without venting your precious heat outside. What about a fan? Even a small one can move moisture from the powder room out to the hallway, and I’m hearing that doorway fans are particularly nice for forcing air from one room to another. Or how about a small dehumidifier, Barb? Its sole purpose is to wring out sopping air, and a small and/or Energy Star-rated machine cuts down on electricity use. Both of these methods require energy to run, unlike that open-door policy above, but they skip the part where your heater has to fire up again.

All this said, it’s not a terrible idea to run the exhaust fan, Barb, if you do it sparingly: Try 5 or 10 minutes after your shower, then flip the switch. Even better if your fan is Energy Star-approved and correctly sized for your bathroom – too much power just means wasted electricity. It’s a lot more efficient than opening a bathroom window in February, anyway.

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I should mention that there are home technologies out there that do an even better job of dealing with shower steam and more: Heat recovery ventilators (HRVs) and energy recovery ventilators (ERVs) are whole-house systems that transfer most of the heat (and water vapor, in the case of ERVs) from the outgoing air to the new air they pull inside to replace it. These systems can even be set to vent from rooms like the bathroom and kitchen and to pump the fresh stuff into bed- and living rooms. HRVs/ERVs are best for very tight – that is, energy-efficient – houses and can be quite expensive, especially in certain climates, but they’re worth a look if you’re ever in the market to remodel.

Otherwise, Barb, you can speed up the bathroom-drying process by hanging wet towels on the outside of the door, not inside. And though I know this sounds like torture in those cruel Midwestern winters, a shorter, cooler shower will also help keep the steam levels manageable (and save still more energy, plus water to boot!). Hey, the Scandinavians swear by it.


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