A bathroom extract fan has to actually move air to outside, and it has to run long enough. Test it with a sheet of tissue — the fan should hold it against the grille. Run it during the shower and for 15 to 20 minutes after, ideally on a timer. If the fan is weak, the duct is long, crushed or disconnected, or it vents into a ceiling void instead of outside, it needs fixing — that is the root cause of recurring bathroom-ceiling mold.
Key takeaways
- The tissue test: a working fan holds a single sheet against the grille.
- Run the fan during and for 15 to 20 minutes after showering — a run-on timer is ideal.
- A fan ducted into the ceiling void, not outside, does almost nothing.
- A windowless bathroom with no working fan will keep growing mold.
Test your fan
Turn the fan on and hold a single sheet of toilet tissue flat against the grille. A properly working fan holds it there. If the tissue barely moves or falls away, the fan is not extracting effectively — and the moisture from your showers is staying in the room. Also check: does it come on with the light, on its own switch, or on a timer? Does it keep running after you leave?
Common ventilation faults
- Undersized or old fan — not enough extraction for the room.
- Long, crushed or disconnected flexible duct in the ceiling void.
- Ducted into the void, not to outside — the moisture just moves into the ceiling space and can cause problems there instead.
- Blocked external grille or a backdraught shutter stuck closed.
- Never switched on, or switched off as soon as you leave.
Improving it
- Fit a fan rated for the room size, ideally with a run-on timer (keeps going 15–20 minutes after the light is switched off).
- Have the duct route checked — short, smooth and terminating at an external grille.
- Use the fan every time, during and after showering, and leave the door open between uses.
- Squeegee the shower glass and wipe surfaces after use to cut the moisture load.
Windowless bathrooms
An internal bathroom with no openable window depends entirely on mechanical extraction. If the fan does not work properly, there is no fallback — the room cannot dry. This is the single most important thing to get right, and it is worth paying for a proper fan and a verified duct run.
If a bathroom fan vents into the ceiling void rather than to outside air, all it does is move your shower moisture into the roof space. Have the duct termination checked.