TL;DR
A bathroom fan that no longer clears steam is almost always losing airflow to one of three things: a blower wheel packed with dust, a backdraft damper stuck shut, or a crushed or disconnected duct. Test it with a single square of toilet paper held to the grille. If the paper falls, the fan is moving far less than the 50 cubic feet per minute a bathroom needs.
How Do You Test Bathroom Fan Airflow?
Hold a single square of toilet paper flat against the running fan grille and let go. A fan with healthy airflow pins the paper to the grille and holds it there. If the paper flutters or falls, airflow has dropped well below what the room needs and something downstream is restricting it.
That test takes five seconds and tells you whether to keep reading. For reference, a bathroom needs at least 50 cubic feet per minute from a fan used intermittently, or 20 cubic feet per minute if it runs all the time. Source: International Residential Code ventilation provisions
Cause One: Dust on the Blower Wheel
This is the most common cause and the easiest to fix. The blower wheel is a small drum with curved vanes, and each vane fills with a felted layer of dust and airborne body oil. Once the vanes lose their shape, the wheel stops grabbing air.
A grille that looks gray is a strong hint, but the grille is only the visible part. The wheel behind it is usually far worse, and the fix is a full cleaning rather than a wipe. That process is covered step by step in cleaning the fan cover and blades, starting with switching off the breaker rather than the wall switch.
Cause Two: A Stuck Backdraft Damper
Nearly every bath fan has a flap that opens when the fan runs and falls shut when it stops, so outside air does not blow back in. There is usually one at the fan housing and another at the exterior wall or roof cap.
Lint, dust, paint, or a bird nest can hold that flap shut or partly shut. A damper that only opens a crack cuts airflow dramatically no matter how clean the wheel is.
Go outside and look at the exterior cap while someone runs the fan. You should see the flap swing open. If it does not move, or if the screen behind it is matted with lint, that is your problem. Clearing an accessible exterior cap is a homeowner job; a roof cap is not, and hiring that out typically runs $75 to $200 for a service visit.
Cause Three: The Duct Itself
Flexible duct is the weak point in most installations. It sags between joists, kinks around a tight turn, gets crushed under stored boxes, or simply pops off the fan collar and dumps humid air into the attic.
Long runs and multiple elbows also cost airflow. A fan rated at 80 cubic feet per minute on the box is measured against light resistance, and a real duct run with two elbows and twenty feet of flex delivers a good deal less than the label. Source: Home Ventilating Institute fan rating guidance
Two things are worth checking from inside the attic if you can get there safely. First, is the duct actually connected at both ends. Second, does it terminate at an outside cap rather than just ending under the roof. A duct that vents into an attic is a code violation and a moisture problem for the whole house, not just the bathroom.
Cause Four: No Air Coming In
A fan can only push out as much air as it can pull in. A tightly weatherstripped bathroom door with no undercut starves the fan, and you will hear the difference the moment you crack the door. The usual fix is trimming the door bottom to leave about a three quarter inch gap above the finished floor.
Cause Five: The Fan Was Always Too Small
Sometimes nothing has failed. A 50 cubic feet per minute fan in a large bathroom with a separate shower and a soaking tub was never going to keep up.
A common sizing rule of thumb is one cubic foot per minute for each square foot of floor area in bathrooms up to 100 square feet, and larger bathrooms are sized by counting fixtures instead. Source: Home Ventilating Institute sizing guidance If the room is big and the fan is small, cleaning will help but it will not solve the problem.
When It Is Time to Call Someone
Call an electrician or a ventilation contractor if the fan hums but the wheel does not turn, if it grinds or squeals after cleaning, if the housing is hot, or if you smell burning. Those are motor and wiring symptoms, not cleaning symptoms.
And if the ventilation problem has already produced growth on the ceiling or walls, understand what cleaning can and cannot do. Wiping a surface does not fix the moisture source that fed it, which is the same point made in why shower grout turns dark and in removing mold from a bathroom. Extensive growth is a remediation trade, not a cleaning job.
FAQ
How can I tell if my bathroom fan is working properly?
Hold one square of toilet paper flat against the grille while the fan runs. Healthy airflow holds the paper against the grille without your help. If it flutters or drops, airflow is too low and the cause is usually a dusty blower wheel, a stuck damper, or a bad duct run.
Why does my bathroom mirror stay fogged even with the fan on?
The fan is not moving enough air to replace the humid air fast enough. Start with a full cleaning of the blower wheel, then check that the exterior damper opens and that the duct is connected and runs outdoors. An undersized fan in a large bathroom will also fog the mirror.
Does cleaning a bathroom fan actually improve airflow?
Often dramatically. Dust felts into the curved vanes of the blower wheel and destroys its ability to grab air, so removing it restores much of the original performance. Cleaning will not help if the real restriction is a stuck damper or a crushed duct.
How long should a bathroom fan run after a shower?
Run it during the shower and for roughly twenty minutes afterward. The trailing run is what actually removes the moisture that has already been released into the room. A timer switch makes this automatic and is the single most useful upgrade for a damp bathroom.
Is it bad if my bathroom fan vents into the attic?
Yes. Warm, humid air dumped into an attic condenses on cold framing and sheathing, which leads to damp insulation and wood problems over time. Codes require bathroom exhaust to terminate outdoors, so a duct ending in the attic should be corrected by a contractor.
Can a bathroom fan be too small for the room?
Yes. A rough sizing rule is one cubic foot per minute per square foot of floor for bathrooms up to 100 square feet, with larger rooms sized by fixture count. A small fan in a large bathroom with a separate shower and tub will never clear steam quickly.
When it stops being one job and becomes every room, house cleaning services are the wider answer.
Ventilation is the master variable in a bathroom, and it is worth fixing before you spend money on stronger cleaners, as explained in whether a bathroom fan prevents mildew and controlling humidity in a bathroom with no window. If the room needs a full reset once the airflow is sorted out, a professional cleaning gets you back to a baseline you can hold. 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.