Damp and Mould in the Home

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Many homes have minor problems with mould on window sills often caused by condensation on windows on cold mornings. However more extensive damp or mould can be a serious problem for the structure of the building, your clothes and your furniture and for your health. Mould spores can be a serious irritant to the lungs.

What causes damp?

Dampness can have any of three causes

  • Condensation - due to moist air hitting cold surfaces and releasing water
  • Rising damp - due to damp from the ground
  • Penetrating damp - from rain or plumbing failures

What does rising damp look like?

Rising damp can affect any wall in contact with the ground and therefore can affect internal as well as external walls. It normally does not rise above about 1 metre in height and you will usually see a characteristic ‘tide mark’. Below the tide mark the plaster below feels cold or damp to the touch. In other respects it can look similar to penetrating damp


Rising damp with characteristic tide mark - picture from www.safeguardeurope.com

This is a damp proof course in a victorian house. You can just see a thicker layer of mortar with 2 layers of slate in it. Sometimes lead is used instead. DPCs are not always like this though. Sometimes they look like a coloured mortar.

How do I fix rising damp?

Rising damp is caused by a problem with your damp proof course. Either

  • you haven't got a damp course
  • your damp course is cracked or otherwise damaged - for example by tree roots or ground movement
  • the damp is bypassing the damp proof course - for example, if earth has banked up against the wall to a level higher than the damp course, then the damp can get in above it.

In the last case, you need to remove the material that is bridging the course. Otherwise, you need a specialist to fix the damp proof course.

What does penetrating damp look like?

This type of damp can affect almost any location in the home and is usually the result of a building or plumbing fault allowing water to enter e.g., through holes in the roof; spilling from blocked guttering; around window frames; through cracked/leaky pipes; overflows. If you think you have a problem with penetrating damp it is advisable to have the fault rectified as soon as possible. With more extreme weather, heavy, wind-driven horizontal rain can penetrate right through mortar joints to the inside face of the wall.

If the damp comes through to the inside of a plastered wall then you can get 'blown' plaster. This happens because water in the wall dissolves salts in the material and then when it evaporates off the surface the salts crystalise out. The crystals push the plaster out of the way. You will need to fix the source of the problem, wait for the wall to dry and then replaster.


Blown plaster - picture from www.diydoctor.com

What does condensation look like?

Condensation usually occurs during cold weather, whether it is raining or dry, and it does not leave a tidemark. It appears on cold surfaces and usually forms in places where there is little movement of air, e.g. corners of rooms; on or near windows; in or behind wardrobes/cupboards/window blinds; on north-facing walls. It usually affects properties during the months of October to April when home ventilation is at its lowest. During this period you tend to close windows and doors, thus allowing moisture levels to build up.


Condensation problems usually lead to mould as on this wall next to the window. The picture is from www.brighton-hove.gov.uk

How do I manage condensation?

Condensation is caused by a combination of high moisture levels and/or cold areas.

To reduce moisture levels you need to reduce moisture sources and ensure you have good ventilation. Moisture can come from:

  • Cooking: cover pans and do not leave kettles boiling.
  • Washing: dry washing outdoors on a line or put it in the bathroom with the door closed and the window open or fan on. If you have a tumble dryer make sure it is vented to the outside, unless it is the self-condensing type.
  • House plants that are very thirsty - most of the water you give them ends up in the air of the house
  • People and animals - when we breathe we generate moisture. The more people you have the more ventilation you need.

So you need ventilation but you don't want draughts. Our ventilation page has more advice on this, but here are some suggestions

  • Use shock ventilation (a loose translation from the German 'Stoß-Lüften'. Open all your windows, and doors between to encourage a through draught and get a complete change of air - just for five minutes. Do this two or three times a day, as often as you feel the need. The idea is to change the air quickly, not leaving time for the walls and structure of the house to cool down too.
  • Keep your windows slightly open or a trickle ventilator open. This ensures some air flow at all times.
  • In kitchens and bathrooms, use an extractor. Keep the door closed when they are in use, to stop the steam escaping into the rest of the house. Leave the bathroom fan running for a while after you have finished.
  • Squeegee showers after use.
  • Allow space for air to circulate in and around furniture.
  • Make sure wardrobes and cupboards have ventilation and are not stuffed so fill that the air can't circulate. Position them against internal walls where possible.
  • When you close your curtains or blinds on a cold night the window surface can get very cold. Trickle ventilators can help reduce the problem.

The other approach to fixing condensation is to avoid cold areas. That means keeping the whole house warm. Insulation will make this easier - make sure there aren't any gaps. See our insulation advice page. However:

  • Do not block ventilation around the eaves in your loft
  • Do not draught proof rooms where there is a fuel-burning heater such as a gas fire, cooker or stove
  • Do not draught proof windows in the bathroom or kitchen.
  • In cold weather keep your heating on a low setting all day.

How should I get rid of the mould?

If you have mould you need to treat it as well as fixing the cause.

First wipe down walls and window frames with a fungicidal wash, which carries a Health and Safety Executive approval number. Follow the instructions precisely. Dry-clean mildewed clothes and shampoo carpets. If you disturb the mould by brushing or vacuuming this can increase the risk of respiratory problems.

After treating the mould you will need to redecorate. For paintwork it is a good idea to use a fungicidal paint but this won't work if you cover it with ordinary paints and wallpaper. If you are wallpapering, use a paste containing a fungicide.