Bathroom condensation and mould usually mean steam is not getting out fast enough. The most common causes are a fan that is too weak, switches off too soon or blows into the loft, plus cold spots on outside walls and window reveals. Drying washing indoors makes it worse. The safe first step is simple: run the fan and open the window during and after every shower, then check whether the fan actually pulls air.
Is bathroom mould dangerous, and what should I do right now?
Mould is not an emergency, but deal with it soon. It can irritate the lungs and make asthma and allergies worse, especially for babies, older people and anyone with a weak immune system.
Right now: open the window, leave the fan running after showers, and wipe water off tiles and glass. If water drips from a light fitting or a fan, switch off at the fuse box and treat it as a leak.
Is it condensation, damp or a leak?
Condensation is the most likely cause, but rule out the others first. Where the mould sits and when it gets worse usually tells you which it is.
- Condensation
- Black spotty mould high up, in corners and on reveals. Worse in winter and after showers.
- Rising damp
- A tide mark low on a ground-floor wall, often with white salty powder. Rarely black mould.
- Penetrating damp
- A patch at any height on an outside wall that grows after heavy rain.
- A leak
- A stain near pipes, the bath or shower, or on the ceiling below. Weather makes no difference.
Why is my extractor fan not stopping the condensation?
Usually because it moves far less air than it should. Approved Document F sets a minimum of 15 litres per second for a bathroom fan, and many older fans never managed that.
Hold a sheet of toilet paper to the grille with the fan running. If it is not pulled firmly on, airflow is poor. Common reasons:
- Too weak for the duct
- Long or bendy ducts cut airflow, especially with a slim axial fan.
- Switches off too soon
- A fan on the light switch stops when you leave, with steam still in the air.
- Ducted into the loft
- Damp air goes into the roof space instead of outside. Check in the loft.
- Blocked or stuck
- Dust on the blades, a squashed duct or a jammed outside shutter.
- Worn out
- Noisy, slow fans with tired motors often move very little air.
Why does mould grow in some spots and not others?
Because condensation forms on the coldest surfaces first. Warm, wet air from a shower meets a cold wall, cools down and drops its water there.
In many Leicester homes, especially older solid-wall terraces, the coldest spots are the outside wall, the ceiling corners along it, and the window reveals (the plastered sides of the window opening). A bathroom heated only occasionally runs colder than the rest of the house.
Which everyday habits make bathroom condensation worse?
A few small habits make a big difference, and they cost nothing to change.
- Drying clothes indoors
- A load of washing releases a lot of water into the air.
- Closing the window straight after a shower
- Leave it open for a while, with the fan running.
- Leaving the door open while showering
- Steam spreads to colder rooms. Keep it shut, then air the room.
- Turning the fan off because of noise
- A quieter, better-matched fan solves this properly.
How do I clean bathroom mould safely?
Treat it gently and protect yourself. NHS advice is to wear rubber gloves, goggles and a mask, open the window and keep the door shut so spores do not spread.
Use a mould spray made for the job, following the label. Never dry-brush or scrape mould, as that throws spores into the air, and never mix bleach with other cleaners. Wipe dry, then bin the cloth.
Mould inside silicone sealant will not clean off, so the sealant needs replacing. If mould covers a large area or keeps coming back within weeks, get the cause looked at.
When should I get a new fan, and what will it involve?
If the paper test fails, the fan blows into the loft, or mould returns soon after cleaning, start with the fan. Most bathrooms need a single visit.
An electrician checks the fan, duct route, wiring and outside vent, then fits a fan matched to the room and duct length, ducted outside and wired with a fused isolator. In a room with no opening window, Approved Document F calls for a 15-minute overrun.
A timer or a humidistat (a sensor that switches the fan on when the air is damp) means nobody has to remember. Humidistats suit family homes and rentals. Bathroom electrics have zone rules and some work is notifiable under Part P, so this is not a DIY job.
How do I stop condensation and mould coming back?
Keep moisture moving out and surfaces warm.
- Fan on every time
- During the shower and for 15–20 minutes after, or let the timer do it.
- Wipe down
- A quick squeegee of tiles and glass removes a lot of water.
- Keep the room warm
- Low, steady heat in winter keeps walls above dew point.
- Clean the fan grille
- Every few months, with the fan isolated first.
- Check the outside vent
- Make sure the flaps open and nothing blocks them.
Questions
Who is responsible for bathroom mould in a rented home?
Broadly, the landlord deals with the causes, such as a broken or missing fan, leaks and damp, while tenants use the ventilation provided and report problems. Damp and mould is a hazard under HHSRS. Awaab's Law covers social landlords and is expected to extend to private renting, so check current guidance.
Will a dehumidifier fix bathroom condensation?
It helps, especially in a home where you dry washing indoors, but it treats the symptom. In a bathroom, steam is best removed at source by an extractor fan that is ducted outside. A dehumidifier is a useful extra, not a replacement.
Can mould-resistant paint solve the problem?
It can slow mould down on walls and ceilings, which is useful in a bathroom. It does not remove the moisture, so mould will still grow on grout, sealant and anything else that stays damp. Fix the ventilation first, then repaint.
Should my bathroom fan run all the time?
Some fans are designed to. Continuous-running fans tick over quietly all day and boost when the room is used. A standard fan is meant to run during and after use, ideally with a timer or humidistat so it does not stop too soon.
What photos help when asking for a fan quote?
The bathroom, the current fan and any label on it, the mould, the outside wall where a vent would go, and the loft above if the fan is ducted that way. With those, we can usually give one fixed price before anyone visits.