Bathroom condensation and extractor fans
A steamy bathroom after a shower is normal. Mould on the ceiling, black grout and a mirror that's still fogged an hour later aren't. The difference is almost always how well — and how long — moisture is extracted.
How much moisture a bathroom deals with
A shower releases roughly 0.2–0.5 litres of water vapour into a room that may be only 10–15 cubic metres. Within minutes the air is saturated and water condenses on every cool surface: mirror, tiles, window, ceiling. A household taking several showers a day, plus towels drying on the rail, keeps the bathroom wet for much of the day unless the moisture is removed quickly.
Is your extractor fan actually working?
Many fans move far less air than their label suggests, because of long or kinked ducting, blocked grilles, weak backdraught shutters or simple age. Two quick tests:
- The tissue test. Hold a single sheet of toilet paper flat against the grille with the fan running. A working fan should hold it against the grille. If it flutters away or barely moves, airflow is poor.
- The clearing test. After a shower with the door shut and fan on, time how long the mirror takes to clear. If the room is still foggy after 20–30 minutes, the fan isn't coping.
Also check where the duct goes. Fans should discharge outside. A fan that blows into the loft simply moves the problem upstairs — see loft condensation.
How powerful should a bathroom fan be?
England's ventilation guidance (Approved Document F) sets minimum extract rates for new and replacement fans. For a bathroom, that's typically around 15 litres per second for an intermittent fan, or a lower continuous rate (around 8 litres per second) for fans designed to run all the time. Scotland, Wales and Northern Ireland have their own guidance with similar intent. Check the current document for your nation if you're installing one.
The rate that matters is what the fan achieves once installed, not the number on the box. Short, straight, rigid ducting with few bends performs best.
How long to run it
- Run the fan throughout the shower and for at least 15–20 minutes afterwards.
- Keep the bathroom door closed so the fan pulls moisture out rather than letting it spread to bedrooms.
- Make sure replacement air can get in — usually through a gap under the door (around 1 cm). Without it, the fan can't move much air.
- Overrun timers or humidistat fans do this automatically. Fans tied to the light switch often stop the moment the light goes off.
- Continuous low-speed fans (often called dMEV) run quietly all the time and boost when needed. They're a good upgrade in bathrooms with persistent mould.
Other fixes
- Squeegee the shower screen and tiles after use; it removes water that would otherwise evaporate slowly for hours.
- Leave the window open briefly after a shower if there's no fan, with the door shut.
- Keep the room warm: a heated towel rail or radiator keeps the ceiling and walls above dew point for longer.
- Insulate a cold ceiling under a roof, if loft insulation is missing above the bathroom.
- Treat existing mould safely (see black mould) and replace mouldy silicone — mould grows into it and can't be cleaned out.
Renting? A broken or missing extractor fan is normally a repair for your landlord. See renters' rights.