How do I stop mould coming back?

The moisture, ventilation and temperature changes that stop mould re-establishing after you have cleaned it off.

Difficulty: beginner
Time: 1 hr 30 min
Read: 2 min

Short answer

Mould needs moisture, so removing it permanently means keeping surfaces dry. Reduce moisture production, ventilate properly, and raise the temperature of the coldest surfaces. Keeping indoor humidity below about 60% is the practical target.

Cleaning mould treats the symptom and takes twenty minutes. Preventing it treats the cause and is what actually matters. There are only three levers — how much moisture you produce, how fast you remove it, and how cold the surface it lands on is — and everything below is one of those three.

What you'll need

Hygrometer (optional)
Dehumidifier (optional)
Extractor fan that works

Step by step

Measure the humidity with a cheap hygrometer before changing anything.Sustained readings above 60% mean a real moisture problem worth spending money on.
Cut moisture production: lids on pans, showers shorter, laundry dried outside or vented.A single wash load releases around two litres of water into the air as it dries indoors.
Run extractors during and for twenty minutes after cooking and showering.Most of the moisture leaves the room after you do. Fans wired to the light switch stop far too early.
Ventilate with short, wide bursts — windows open on opposite sides for ten minutes, twice a day.Fast air exchange without cooling the walls, which is what a window on the latch all day does.
Keep background heat on rather than heating in bursts.Surfaces stay above the dew point. Letting a house go cold and then blasting it is what produces condensation.
Pull furniture 5cm off external walls and leave trickle vents open.Airflow across a cold surface stops moisture settling there.
Never block air bricks or vents.They are designed ventilation. Sealing them is a common cause of a sudden new damp problem.
Re-measure after a fortnight and adjust.You are aiming to move the humidity number, not just to feel like you have done something.

Tips

A dehumidifier is worth it in a genuinely damp property, but fix ventilation first — otherwise you are paying electricity to work around a cheaper fix.
Anti-mould paint buys time on a wall that keeps getting damp. It treats the symptom, so use it after fixing ventilation rather than instead.
Bedroom mould is common because a closed room with sleeping people is a humid room. Leaving the door ajar overnight helps considerably.

Common mistakes

Blocking vents to stop draughts — It removes the ventilation path and is a leading cause of new condensation and mould.
Relying on cleaning alone — Mould returns within weeks if the moisture conditions have not changed.
Turning heating off entirely when out all day — Cold surfaces when you return and start cooking is exactly when condensation forms.

Questions people ask

What humidity level prevents mould?

Keeping indoor relative humidity below about 60% makes it hard for mould to establish. A hygrometer costs a few pounds and turns this from guesswork into a measurement.

Do dehumidifiers stop mould?

They manage the symptom effectively and help a wall dry after a leak is fixed. They do nothing about the cause of penetrating or rising damp, which need the water source stopped.