Symptoms
- Fan runs but mirror still fogs up and stays foggy
- Bathroom stays humid well after a shower
- Weak airflow felt at the grille
Likely causes
- CommonFan is undersized for the bathroom's square footageAn underpowered fan can run fine mechanically but still not move enough air.
- CommonClogged or crushed ductworkFlexible ducting can sag, kink, or fill with lint over years, choking airflow.
- PossibleDuct venting into an attic instead of outsideSome fans were improperly installed to dump moist air into the attic rather than outdoors, which also risks mold.
- PossibleA blocked or damaged exterior vent capDebris, nests, or a stuck flapper at the outside vent can restrict airflow.
What to do
- Feel airflow at the grille with your hand; weak airflow points to a duct or fan sizing issue rather than the switch or motor.
- Check the fan's rated CFM against your bathroom's square footage using manufacturer guidelines.
- If accessible, check ductwork in the attic for kinks, sagging, or disconnection.
- Check the exterior vent cap for blockages, debris, or a stuck flapper.
- Confirm the duct actually terminates outside the house and not into an attic space.
- Clean visible dust from the fan blade and housing, since reduced airflow is sometimes just buildup.
- Have an HVAC technician or electrician upgrade the fan or repair ductwork if it's undersized or improperly vented.
Tools and parts
- Flashlight for attic inspection
Higher CFM bathroom exhaust fanSearch: “bathroom exhaust fan CFM rated”
When to call a pro
Call an HVAC technician or electrician if ductwork needs repair or rerouting, or if the fan needs to be upsized.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.