Why Your Bathroom Fan Isn't Clearing Steam
Part of our Ventilation & Hot Water guides.
Hold a tissue under the grille: if it doesn't stick, airflow is weak.
| Time | 30-60 minutes |
|---|---|
| Difficulty | Easy |
| Tools | Screwdriver, vacuum, flashlight |
Step by step
- Clean the grille. Dust buildup chokes airflow.
- Check run time. Run 20 minutes after showering.
- Check the damper. The flap at the fan or outside cap may be stuck.
- Inspect the duct. Crushed, disconnected or long flex duct cuts airflow.
- Check the outside cap. Nests or debris can block it.
- Size. A fan smaller than 1 CFM per square foot will struggle.
- Replace. Old fans lose performance; a new quiet fan is inexpensive.
Tips & common mistakes
- Make-up air: a gap under the door lets air in so the fan can pull air out.
The tissue test
Turn the fan on and hold a single square of toilet paper near the grille. If the fan holds it firmly, airflow is good. If it barely moves, something is restricting airflow.
Duct problems
- Flexible duct crushed above the ceiling or sagging into loops.
- Duct disconnected from the fan housing (it's blowing into the attic).
- Very long runs with several bends; each elbow reduces airflow.
- A roof cap with a stuck damper or screen clogged by lint and debris.
Make-up air
A fan can only remove air if air can enter. A gap of about 3/4 inch under the bathroom door, or a door grille, lets replacement air in. Tight bathrooms with weather-stripped doors starve fans.
When to upgrade
If the fan is old, noisy and undersized, replacing it with a quiet 80-110 CFM unit and a humidity sensor usually solves the problem. Use the CFM calculator to check size.
Frequently asked questions
Why does my mirror still fog with the fan on?
Some fogging during a hot shower is normal; it should clear within minutes after.
Should the fan run during the shower?
Yes, turn it on at the start and leave it running 20 minutes after.
Can a clogged roof vent cause problems?
Yes; birds' nests and debris often block roof caps.
How do I test the backdraft damper?
With the fan off, check the flap at the housing moves freely; outside caps should open when the fan runs.
Is a louder fan more powerful?
Not necessarily; noise often comes from worn motors or poor ducts.