TL;DR
It helps significantly but does not prevent mildew on its own. A correctly sized fan vented outdoors and run for 20 to 30 minutes after a shower pulls out most of the moisture that mildew needs. A fan that is undersized, vented into the attic, or switched off with the light does very little.
Does a Bathroom Fan Prevent Mildew?
It reduces the conditions mildew needs rather than preventing it outright. Mildew grows where surfaces stay damp, and a shower puts a large amount of water vapor into a small room in a short time. The fan's job is to move that vapor outside before it condenses on cold tile, grout, caulk and the ceiling.
Do that reliably and mildew becomes rare. Do it partially -- which describes most bathrooms -- and you slow growth without stopping it. The fan is one of three or four things that have to be right together.
The Four Things That Decide Whether a Fan Works
Sizing. The common rule of thumb is roughly 1 CFM of fan capacity per square foot of bathroom floor area, with 50 CFM as a practical minimum. A 60-square-foot bathroom wants at least a 60 to 80 CFM fan. Bathrooms with a separate enclosed toilet, a jetted tub or a high ceiling need more.
Source: Home Ventilating Institute sizing guidance
Where it vents. This is the failure that ruins everything else. A fan ducted into the attic or a soffit cavity does not remove moisture from the house -- it relocates it into insulation and framing, where it causes a much larger problem. The duct must terminate outdoors, through the roof or a wall, with a damper.
Run time. Twenty to thirty minutes after the shower ends, not thirty seconds. The vapor does not leave with the last of the hot water; it clings to warm surfaces and releases as they cool. This is the single most common gap, and a timer switch or a humidity-sensing switch solves it permanently.
Makeup air. A fan cannot exhaust air it cannot replace. With the door closed and no gap underneath, a fan pulls against a sealed room and moves a fraction of its rated airflow. An undercut of about three-quarters of an inch at the bottom of the door, or leaving the door ajar, restores it.
What the Fan Cannot Do
It cannot fix cold surfaces. An exterior wall or a poorly insulated ceiling in an unheated bathroom will condense moisture faster than any fan removes it. In an Oregon winter that shows up as a shadow of mildew tracing the framing on the ceiling.
It cannot remove water that is not airborne. Standing water on the shower floor, pooled on a flat ledge, or trapped in the track of a sliding door evaporates slowly and feeds growth locally regardless of the room's overall humidity. That is what makes a squeegee against scrubbing such an efficient trade -- thirty seconds removes water the fan would take an hour to handle.
It cannot remove existing growth. A fan changes the conditions going forward. Mildew already established on caulk, grout or paint stays until it is cleaned off, and the approach in removing mold and mildew in a bathroom is what actually clears it.
And it cannot fix a leak. Persistent dampness at a tub edge, around a toilet base or under a vanity is plumbing, not ventilation.
How to Test the Fan You Have
Hold a single square of toilet paper against the grille with the fan running. If it holds firmly, airflow is reasonable. If it falls or barely clings, the fan is undersized, the duct is crushed or disconnected, the exterior damper is stuck, or the grille and blades are packed with dust.
Then check the outside termination on a cold day. You should feel air moving out of it while the fan runs. No airflow at the exterior vent while the fan hums is the classic sign of a duct that came apart in the attic.
Finally, look at the run time habit. If the fan is wired to the light switch, it almost certainly runs for the length of the shower and no longer, which is the least useful possible schedule.
Cleaning the Fan Itself
A bathroom fan grille loads with lint and dust, and a coated blade wheel moves noticeably less air. Cut power at the breaker, pull the grille, wash it in the sink, and vacuum the housing and blade wheel with a brush attachment. Twice a year is a reasonable cadence; more in a busy family bathroom.
This is routine maintenance most people never do once, and the airflow difference on a neglected unit is dramatic.
The Rest of the Bathroom Humidity System
Where there is no fan at all, a window plus a small portable dehumidifier is the working substitute, and the options in humidity in a windowless bathroom cover the range. Keeping the door open after showering helps in any bathroom by letting the moisture disperse into a much larger volume of house air.
Sealing is the other half. Failed caulk is porous and holds water behind it, which is why black mildew on shower caulk keeps coming back after cleaning. Sealed grout sheds water instead of absorbing it. Neither is a ventilation fix, but both change how much water is available to sit.
For anything larger than surface growth on a bathroom's finishes, the distinction in mildew and mold are not the same matters -- growth inside a wall or across a large area is remediation work rather than cleaning, and better ventilation afterward will not undo what is already there.
FAQ
How long should a bathroom fan run after a shower?
Twenty to thirty minutes after the shower ends. Water vapor releases from warm surfaces as they cool, so a fan that stops with the light removes only part of it. A timer switch or a humidity-sensing switch handles this without anyone having to remember.
What size bathroom fan do I need?
Roughly 1 CFM per square foot of floor area, with 50 CFM as a practical minimum. A 60-square-foot bathroom wants 60 to 80 CFM. Add capacity for high ceilings, jetted tubs and separate enclosed toilet compartments.
Can a bathroom fan vent into the attic?
No. Venting into an attic or soffit cavity dumps shower moisture onto framing and insulation, where it causes condensation, staining and growth far worse than anything in the bathroom. The duct must terminate outdoors with a damper.
Why does my bathroom still get mildew with the fan running?
Common causes are a fan that stops too soon, a closed door with no undercut so the fan cannot pull air, a disconnected or crushed duct, cold uninsulated surfaces, or standing water sitting in the shower. Usually more than one is in play.
Does opening a window work instead of a fan?
It helps, but it is weather-dependent and slow, and on a damp day outdoor air brings its own moisture. A window plus a small dehumidifier is a reasonable substitute where no fan exists. A properly ducted fan is more reliable year-round.
How often should you clean a bathroom exhaust fan?
Twice a year is a reasonable baseline, more in a heavily used bathroom. Cut power at the breaker, wash the grille, and vacuum the housing and blade wheel. A dust-loaded fan moves substantially less air than its rating suggests.
A good fan is necessary and not sufficient, and the bathrooms that stay clear are the ones where venting, run time, a squeegee habit and regular house cleaning all line up. Every home is different, so the real number depends on size, condition, and what you want covered. Get a quote and we will walk you through it.