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Condensation, damp and mould

Spores of black mould on a wall made damp by condensation

Damp damages your home and causes mould, which is bad for your health. Here’s how to treat and prevent it.

Last updated, Dec 2023

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:


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.

A window ledge with speckles of black mould.
Mould caused by condensation damp is characterised by black dots, like on this window recess.

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).

A pie chart breaking down the different sources of moist air in a home.
Typical moisture production for a family of four (litre/day)Nicol, F. and Rudge, J. eds., 2005. Cutting the cost of cold: Affordable warmth for healthier homes. Routledge

Condensation occurs due to one or more of the following:

Preventing condensation

Two immediate solutions to condensation are …

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



Reducing moist air

Checklist for homeowners, housing providers and landlords

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 …

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.

Paint flaking away from a wall.
Penetrating damp can cause paint to flake. It can be caused by structural issues like cracked masonry or leaking guttering.

The common causes of penetrating damp are related to the fabric of the building.

This includes damaged roof tiles or felt, defective flashing around chimneys, cracks in the render or pointing between bricks, defective guttering, rotting wood or gaps around doors or window frames, and high external ground levels forcing water through the wall.

Be aware that identifying the cause can be difficult because water can travel horizontally as well as vertically, so the damp patch may not be where the water is coming in to your house.

If you can’t find the cause of the damp, ask a damp specialist to do a survey of your property.

The solutions to penetrating damp are likely to be structural. This may include sealing cracks or gaps in the outside of the walls or roof with appropriate materials, replacing damaged roof tiles, felt and flashings, clearing and repairing guttering, or lowering any high external ground levels, where possible. Sometimes these are DIY jobs, though in other cases you may need a professional builder, roofer or damp specialist.

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.

Rising damp leaves a visible ‘tide mark’ on your skirting board or walls.

The most common cause of rising damp is a defective or non-existent damp-proof course. A damp-proof course is a horizontal barrier, typically made of plastic or bitumen, installed in the walls of a building to prevent moisture from rising through the structure. Some houses did not have damp-proof courses put in when they were built. In other cases, the damp-proof course may have been damaged by subsequent building work. And it is also possible that the the ground outside has been raised to a level above the damp-proof course, for example, when a patio was laid down.

It is often the case that damp is misdiagnosed as rising damp, when it is actually penetrating damp or even condensation. Make sure that you get advice from a specialist who has experience working with buildings of the same age as yours.

While rising damp is the least likely cause of damp, the solutions to it are often the most complex. It may involve signifcant building works, such as installing a damp-proof course, digging a drainage channel around the house, or replacing modern cement and gypsum renders and plasters with vapour permeable materials such as lime.

In all cases, you will need to get advice from a specialist.

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.

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).

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