Bath Fans Can Make Indoor Humidity Worse

Your bathroom exhaust fan clears steam after a shower, but on mild, humid days it can pull more moisture into the house than it removes. This happens because the fan draws in outdoor air to replace what it exhausts, and when the outdoor dew point (a measure of how much moisture is in the air) is high, that incoming air adds humidity rather than reducing it. Cloudy, muggy weather in the 70s or low 80s Fahrenheit is the trickiest case, since the air conditioner doesn't run enough to dry out the indoor air on its own.

The type of switch controlling your bath fan matters. A humidity-sensing switch can backfire in an old, leaky house, since it may need such a high threshold to avoid nuisance runs that it barely helps — and in muggy weather it can end up running for hours, drawing in humid air the whole time. A countdown timer switch, set for a fixed run time after you shower, tends to work better because it limits how long the fan pulls in outside air. Some timer switches offer settings from 15 minutes to 12 hours.

On very hot, humid days, your air conditioner may already be removing enough moisture that the bath fan isn't needed at all. In especially humid regions, running it may do more harm than good, since the moisture it draws in can exceed what it exhausts from the bathroom.