Condensation and mould growth

Content owner : housing@knh.org.uk Last updated : 05/03/11

What is condensation?

Condensation is the dampness formed when air laden with water vapour is cooled by contact with a cold surface.


Why do you get condensation?

If you want to improve your home you must follow these steps :The air we breathe can hold varying amounts of water vapour, depending on its temperature. If warm moist air is cooled by a cold surface, such as a window or external wall, it is then no longer able to hold the same amount of water vapour. The air-borne moisture turns into droplets of water and collects on the cold surface. This is called condensation.


When is it a problem?

Every home gets condensation at some time - usually when lots of moisture and steam are being produced - for example, at bath times, when a main meal is being cooked or when clothes are being washed.

It is quite normal to find your bedroom windows misted up in the morning after a cold night. There is nothing much you can do to stop this.

However, if your home never seems to be free from condensation, read on...

  • Many people believe that the problems they see are the results of damp coming through walls and that it is impossible for condensation to look so bad or create such a mess.
  • However, condensation, which is usually at its worst during the winter, often results in black mould growing on walls and other surfaces.
  • Condensation is surface dampness and mainly occurs on cold walls and windows. But it can also appear in places such as wardrobes next to outside walls, toilet cisterns and water pipes, which may start to drip water even though there are no leaks.

What can you do about it?

The four main ways to deal with condensation are :

1. Produce less water vapour

The amount of condensation depends on how much water vapour is in the air. Many everyday activities add to the water vapour level in your home, but their effect can be kept to a minimum.

Cooking :

  • Cover pans when you're cooking.
  • Don't leave kettles and pans boiling longer than necessary.

Drying clothes :

  • Hang washing outside to dry whenever you can.
  • If you have to use a tumble dryer make sure it's vented to the outside.
  • If you have to dry washing indoors use the bathroom and keep the door shut and the room well ventilated.
  • Do not hang wet washing on radiators all round your home - doing so is very likely to cause condensation problems.

Bathing :

  • Keep the bathroom door shut and the room well ventilated.

Paraffin and some types of gas heaters :

  • Avoid using these sorts of heaters - they are one of the main causes of major condensation problems. Paraffin heaters, portable bottled gas heaters and fixed flueless gas heaters all produce heat, but at the same time they also put a lot of water vapour into the air. One gallon of water is produced by one gallon of gas or paraffin burning.
  • Paraffin and portable bottled gas heaters can also be dangerous and very expensive to run. They can cost as much as, or even more than, heating using peak rate electricity.

2. Don't let it spread

Confine wet air to just a few rooms.


  • Your bathroom and kitchen are 'wet rooms' - keep these doors shut so the wet air can't spread to the rest of your home.
  • Especially when you're washing, cooking or taking a shower or bath, keep the door shut to stop the moist air spreading into the rest of your home.
  • At the same time make sure your bathroom and kitchen are well ventilated so the water vapour can escape outside.
  • This is even more important if some of the other rooms are very cold. If rooms are not being used and are unheated it's a good idea to keep their doors shut.
  • Don't completely draught-proof kitchens, bathrooms and other rooms where condensation is already a problem - you could make it far worse.

3. Keep your home ventilated

Let wet air out! The best way to remove water vapour is by providing adequate ventilation. Nobody likes draughts, but some ventilation is vital.


  • Keep a small window ajar, or a trickle ventilator open, in each occupied room to give background ventilation, (but make sure your home is still secure).
  • Open the windows to let the water vapour out, especially when you're doing the washing or cooking.
  • Windows near the ceiling are more effective at letting water vapour out than ones lower down.
  • Heat recovery fans are very good for ventilating 'wet rooms' such as bathrooms and kitchens. They are more effective than ordinary fans, since they get rid of the moisture from the air and let fresh air in, and also recycle the heat back into your home.

BUT DON'T FORGET - keep your home secure!

  • If you open windows, make sure you shut them again when you go out.
  • If you leave small windows open for background ventilation, make sure they're not accessible from the outside, for example, from a garage or shed roof.

4. Keep your home warm

Heating your home can help solve a condensation problem, but only if it's used in addition to the other three steps already described. However, first of all it needs to be 'dry heat', such as central heating or gas fires, not paraffin or portable gas heaters.

Secondly, simply heating your home will tend to warm the air. Warmer air holds more water vapour, so the air in your home could become even wetter. There'll be more water to condense out onto any cold surfaces. This is more likely to be a problem if you only put the heating on for an hour in the morning and an hour at night. In this case only the air is warmed, and building fabric itself stays cold, so there's more chance of warm wet air being in contact with cold surfaces.

The best approach to heating in order to reduce condensation, assuming you have taken the other three steps, is to heat your home at a low level for a long time. Keep the heating on, but set it to provide just a minimum of background heating. This will warm the whole building up and keep it warm, so there are no cold surfaces.


Dealing with mould growth

The best way of tackling mould is to reduce the condensation levels and prevent it growing in the first place. Dampness from condensation often causes the growth of black mould on walls and other cold surfaces such as tiles. Mould and mildew can also grow on furnishings, curtains and even clothes in wardrobes. It may first appear in corners or behind cupboards, but it can spread across entire walls.

Mould can spoil wallpaper and furnishings and can make your home unhealthy. Mould on washable surfaces can be removed by wiping down with detergents or proprietary mould removers. It can be washed out of fabrics, but may leave stains or spoil colours.

Do not brush or vacuum clean the mould away, as this can increase the risk of respiratory problems. Remember the only way to permanently avoid severe mould growth is to eliminate damp within your home.