Usually the fan is too small for the room, ducted into the attic instead of outside, or running for far too short a time. A fan needs roughly 1 CFM per square foot of bathroom and should run for 20 minutes after the shower stops — not until you leave the room.
Check the three things, in this order
1. Is it actually vented outside?
Go into the attic and look. A distressing number of bathroom fans in Florida homes are ducted to a point six inches above the ceiling and no further — blowing hot wet air straight into the attic, where it condenses on the underside of the roof deck. If yours does that, everything else on this list is irrelevant, and you have a bigger problem than a foggy mirror.
The duct should run to a roof or soffit vent, with as few bends as possible, and it should be insulated. Uninsulated duct in a Florida attic condenses inside itself and drips back down through the fan.
2. Is it big enough?
The rule of thumb is 1 CFM per square foot of floor area, with a minimum of 50 CFM. A 5×8 bathroom needs at least 40, so 50. Add 50 CFM for a separate enclosed toilet, and more for a jetted tub. Most builder-grade fans are 50 CFM and most were fitted regardless of room size.
The number is printed on the fan housing, usually visible once you drop the grille.
3. Is it running long enough?
This is the one nobody gets right. Air in the room is not the problem — moisture in the towels, the walls, the grout and the ceiling is. Twenty minutes after the water stops is the target, and nobody stands around for twenty minutes to switch a fan off.
A timer switch costs about twenty-five dollars, fits in the existing box, and is the single highest-value upgrade in most bathrooms. Set it to thirty minutes and forget it. If your fan is on the same switch as the light, that is worth separating at the same time.
The Florida wrinkle
Running a bathroom fan pulls air out of the house, which pulls replacement air in from somewhere. In a tight modern house that can be the attic, which in August means pulling 130°F humid air into your conditioned space. It is not a reason to skip ventilation — it is a reason not to oversize the fan wildly and to make sure the rest of the house has a sane air path.
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