Condensation and mould in flats

Condensation is the most common cause of damp and mould in flats. It is usually controlled by three things working together: ventilation, steady heating and insulation. Here is how it happens and what you can do about it.

What condensation is

Air always holds some water vapour, and warm air holds more than cold air. When warm, moist air meets a cold surface such as a window, an outside wall or a tiled bathroom, it cools and drops its moisture as water. It is mainly a problem of the colder months.

Everyday life produces more moisture than most people expect. Cooking, showering, drying clothes and simply breathing put several litres of water into the air of a home each day, and a household of five can produce around ten. If that moisture cannot get out, it settles on the coldest surfaces.

Mould

Mould is the most visible result. It starts as black spots on walls, ceilings, window frames and behind furniture, and spreads when surfaces stay damp. It stains decorations and can affect health, particularly for people with asthma, allergies or other breathing conditions.

Wipe mould away with a mould and mildew cleaner, following the instructions on the product. Anti-mould paint can help stop it coming back. The only lasting cure is to reduce the moisture.

Everyday steps that work

• In cold weather, keep the heating ticking over at a low level all day. A flat that is allowed to go cold takes a long time to warm up, and cold walls attract condensation.
• After a bath or shower, open the window or run the extractor fan and keep the door shut, so the steam goes outside and not into the rest of the flat.
• Cover pans when cooking and use the cooker hood or extractor.
• Hang washing outside whenever possible. If you cannot, dry it in one room with the window open or a dehumidifier running and the door closed.
• Keep trickle vents on windows open.
• Leave a gap between furniture and outside walls, and do not overfill wardrobes and cupboards, so that air can move.
• Do not use paraffin or bottled gas heaters. They put a lot of moisture into the air and are usually banned in flats in any case.

If it keeps coming back

• Fit an extractor fan with a humidity sensor in the kitchen and bathroom, so it runs by itself when the air is damp.
• Run a dehumidifier. A good one will collect a litre or more of water a day.
• Improve the glazing. Secondary glazing is a cheaper first step, and double glazing is very effective.
• Add insulation to cold walls and ceilings where that is practical.

Check before you start any of these. Windows, outside walls and the structure often belong to the building and not to the flat, so you may need consent from the landlord or management company. Listed buildings and conservation areas may need council consent too.

New homes

A newly built flat holds a lot of moisture in its plaster, mortar and concrete. It can take 9 to 18 months to dry out, and condensation is common during that time. Gentle, steady heating and good ventilation help it along.

When it is not condensation

Some damp comes from the building, not from the air inside your home. Signs include a damp patch that gets worse after rain, brown staining on a ceiling, a tide mark low down on a wall, or damp next to a leaking gutter or downpipe. If you see any of these at a development we manage, report it through our repairs portal at butlins.fixflo.com with photographs, so we can check the roof, gutters, pipework and outside walls.

If you let your flat

If you rent your flat out, your tenant should report damp and mould to you, and you should take it seriously and act quickly. Private landlords already have to keep a let home fit to live in, and the council can act on serious damp and mould. Stricter legal time limits apply to social landlords under Awaab’s Law, and the government plans to extend them to private landlords. Our article on Awaab’s Law explains more.

This article is general guidance based on the law in England in October 2026. It is not legal advice. Your lease or transfer deed always comes first, so check it, and take professional advice on anything in dispute. You will find more answers in our FAQs.