As of mid-September 2026, in my Port St. Lucie house, the bathroom exhaust fan used to get flipped off the second the shower handle turned off. The mirror was clearing, the water had stopped, so what was left for the fan to do? Florida’s health department lists showers right alongside basements as places where humidity runs high and mold grows, and I still treated the fan like a light switch.
The northern house logic does not work here. I have been climbing into this attic since I moved down from New Jersey, and the one thing Florida has taught me is that humidity does not play by the rules I grew up with. Cutting the fan when the water stops assumes the job is finished, and that is not what the guidance says.
What the Building America Solution Center says
The Building America Solution Center’s guide to bathroom exhaust fans doesn’t split the difference on timing: “It is recommended to leave the fan running for one hour after a shower for effective humidity control.” One This Old House general contractor does the same in his own house, with timer switches that let the bathroom fans run a full hour after use.
Why the extra time matters
The Florida Department of Health lists poor venting of bathroom steam as one of the sources of moisture that feed mold, along with condensation from indoor humidity that is too high or surfaces that are too cold. Its bottom line is simple: “Controlling moisture is the key to stopping indoor mold growth, because all molds require water to grow.”
The Building America guide says exhaust fans are recommended in bathrooms to remove excess moisture, and its timing advice is the full hour after the shower, not just the minutes the water is running. This Old House’s ventilation guide spells out what that protects: bathroom ventilation maintains proper humidity levels, preventing condensation on surfaces and reducing the risk of mold and mildew growth that can damage the structure of your home…