A damp home is a breeding ground for mould, which damages walls and furniture and is bad for your health. Mould spores occur naturally in the air and they quickly multiply once they find a damp surface.

Nearly all damp in homes is caused by condensation. This page covers:

• The causes of condensation damp.
• How to prevent condensation.
• How to treat mould in your home.
• Other causes of damp in the home.
• Dehumidifiers and hygrometers.

Condensation

Condensation is the most common cause of damp. It’s easily recognised by the black mould it causes —small black dots that appear near cold surfaces like window frames.

Causes of condensation damp

When moist air reaches cool surfaces – such as walls, ceilings, windows and pipes – it turns back into water (condenses) making those surfaces damp.

We produce a lot of moisture through our everyday activities at home, such as showering, cooking, drying laundry and even just breathing (see diagram below).

Condensation occurs due to one or more of the following:

• Inadequate ventilation and a build up of excessive moisture.
• Missing or incomplete insulation resulting in cold spots.
• Inadequate heating resulting in lower surface temperatures.

Preventing condensation

Two immediate solutions to condensation are …

• Produce as little moist air as possible — for example, by drying clothes outside and keeping lids on pans.
• Make sure any moist air you create can leave your home through vents, extractor fans and open windows.

The majority of damp issues are caused by condensation and can be rectified by doing things differently in your home. Below is a comprehensive checklist of changes you can make to prevent condensation.

Preventing condensation — checklist

Heating

• Make sure that the heating is working well. It may need a service or repair.
• During colder times of year, turn your heating on regularly. This doesn’t mean all day, every day, but often enough to keep the home dry. If you can’t afford this, then make sure you do as many of the things below as possible …

Ventilation

• Regularly open external doors and windows throughout the home.
• Open the kitchen and bathroom windows to allow the steam created by cooking, showering or having a bath to leave the building.
• Check your existing extractor fans are clean and working correctly. Make sure they turn on automatically or are turned on every time steam is being produced.
• Make sure trickle vents on windows are kept open. Or permanently leave bathroom and kitchen windows slightly open (using the security latch).
• Make sure that the vents and airbricks aren’t blocked or covered up. If the vent has a cover, move it to the open position.
• If you notice mould behind sofas or other furniture, move them away from the walls to allow air to circulate around the back.
• If there are lots of people or pets in a small space, increase the ventilation; we produce a lot of moisture just by breathing!
• Use a humidity meter (also called a hygrometer) to find out which rooms need to be ventilated regularly.

Reducing moist air

• When cooking, keep lids on pans to trap the steam in the pot. This also reduces the cooking time.
• Keeping doors closed will stop the moisture from moving around the home, especially your bathroom and kitchen doors when you’re showering or cooking.
• If you can, avoid drying laundry inside.
• If it’s not possible to dry laundry outside, use the spin cycle on the washing machine twice to reduce the amount of water held in the fabric. Use a tumble dryer or hang the washing in a room with the internal door closed and a window open.
• Make sure clothes are completely dry before putting them in wardrobes and drawers.
• Do not use portable gas heaters indoors as they make the air very moist.
• In severe cases, use a dehumidifier.

Checklist for homeowners, housing providers and landlords

• Insulate the property. This can increase the inside temperature so the air can hold more moisture before it condenses and you’ll be less likely to have damp and mould. For more information on insulating your property, see our factsheets on loft, cavity wall, solid wall and floor insulation.
• Ensure there are extractor fans in the kitchen and bathroom. Ideally these should be mechanical extract ventilators, which run at a low level all the time and respond to higher humidity levels. For more information on increasing ventilation in your property, systems see our factsheet on ventilation.
• Install trickle vents on the windows if the current windows do not have them.
• Make sure there is a kitchen and bathroom door that can be shut to stop the moist air getting into the rest of the house. Also check there is a gap of around 1cm below all other internal doors to allow air to circulate around the house.
• Ensure that the heating system is serviced regularly and necessary repairs are carried out.
• Where possible, provide outdoor space for drying clothes.

Treating mould

The tips above will help you prevent mould growth. But if you don’t treat and clean mould that is already present in your home, it will grow and spread to other areas. Mould spores can exist in walls up to a meter around the visible patch.

Treat mould by following these four steps:

1.	Treat the visible mould with a spray containing bleach and allow it to dry, then repeat.
2.	To kill the invisible mould spores, treat an area at least 1m wider than the mould patch with a spray containing bleach.
3.	Repeat the treatment every time it reappears. It can be frustrating and time-consuming, but it will be worth it to stop the mould from getting worse.
4.	Dispose of furniture, soft furnishings and wallpaper that are too badly affected for treatment to be effective.

Other types of damp

Some damp is also caused by …

• Moisture entering your home through cracks or damage to the exteriors — penetrating damp.
• Moisture from the ground rising through the walls of your home — rising damp.

Penetrating damp

The second most likely cause of damp is penetrating damp. This is where moisture comes through the walls, roof, floor, doors or windows. Penetrating damp may get worse when it rains, especially if it’s windy at the same time.

Rising damp

Rising damp is the least likely cause of damp. This is where moisture rises from the ground through the walls of the home. This will leave a ‘tide mark’ on your skirting board or walls.

Dehumidifiers and hygrometers

Dehumidifiers reduce the amount of moisture in the air. They can help resolve condensation issues in your home, especially if you can’t follow all of the actions on the checklist.

• Set your dehumidifier to under 60% humidity if possible.
• On warm, dry days, opening a window will be just as effective as running a dehumidifier.
• If you close the doors and windows in the room where the dehumidifier is on, it will work more efficiently.
• Buy a dehumidifier that will remove at least 5 litres of water every 24 hours. Avoid smaller, cheaper models. Decent dehumidifiers start at around £100 and cost around 17p per hour to run (based on a price for electricity of 27p per kWh, November 2023).

Hygrometers measure how moist the air is, known as the humidity level. 100% humidity is extremely wet; 0% is completely dry. Mould grows at around 60% humidity and above, so to stop mould forming, try to keep the humidity level below this.

They can be bought for as little as £8.00 and can be used to show what causes humidity in your home to rise (drying laundry inside, cooking etc) and what causes it to fall (opening windows, turning on extractor fans etc).