Mould and condensation

Mould are condensation are both caused by high relative humidities within your house – this is rarely caused by dampness in the walls etc. If the relative humidity anywhere within your house is regularly above 70%, mould will grow on the adjacent surfaces. If the relative humidity continues to increase and reaches 100%, condensation will occur on the adjacent surfaces.

If the relative humidity continues to increase and reaches 100%, condensation will occur on the adjacent surfaces.

Relative humidity is determined by the amount of water vapour in the air and its temperature. The higher the temperature or the lower the amount of water vapour, the lower the relative humidity.

Common causes of high relative humidity

Different parts of your house are different temperatures - even within the same room. For example, outside walls are cooler than inside walls, windows are cooler than outside walls, corners are cooler than their surroundings, etc. Hidden construction details also affect the temperature of different parts of your rooms. An example of this is the wall over your doors and windows. There is normally a hidden lintel or beam over every outside door and window. If it is wooden, that bit of the wall will be warmer and if it is steel or concrete, that bit of the wall will be colder. You won't be able to feel these temperature differences. If you have access to a special thermal imaging camera for houses though, you can actually see the temperature differences - which can be very marked.

When the air in your room touches any surface, it is either warmed or cooled to match the temperature of what it touches. This changes the relative humidity next to the surface. If the surface cools the air, the relative humidity can rise enough to enable mould to grow. If the air already contains a large quantity of water vapour, even modest cooling can be enough to trigger condensation.

How to reduce high relative humidities

You need to adopt a two part strategy:

  • raise the temperature of the areas/surfaces where mould or condensation occurs;
  • reduce the amount of water vapour present in the air of the house.

There are a variety of ways to raise the temperature, depending on where the mould or condensation is. Keeping the windows in the affected room closed, turning heating up, or keeping the heating on for longer are obvious ways to warm the whole room generally. A less obvious possibility is rearranging the furniture - if the mould or condensation is only behind or above certain items of furniture, it is likely to be caused by the furniture stopping the walls getting warm. Similarly with mould and condensation inside cupboards - leave the doors open so the cupboards can warm up. Other things to consider are the possibility of adding some sort of insulation in at least the affected area. Just adding polystyrene insulation when decorating can be enough if the problem is just a cold corner or a cold window lintel.

Reducing the amount of water vapour in the air typically involves very simple changes. Major sources of water vapour in the house are cooking, showering, bathing, drying clothes and tumble drying. When doing any of these, you ideally want a mechanical air extractor running in the same room. (A tumble dryer should vent straight outside as it is really a mechanical extractor in its own right.) If a mechanical air extractor is not present, then you will need to rely on opening a window slightly for a few minutes. Many people make the mistake of opening the window wide and keeping it open. A wide open window just cools the room which you don't want. Similarly, if the window is left open for too long you again cool the room off. Only a small opening is needed for the amount of water vapour in the room to quickly equalise itself with that outside - but you are reliant on the weather. Opening the window on a cool misty wet day won't make much difference!

The other big source of water in the house is... people breathing. To stop water building up in the air when your family are in the house, it is important that some slight ventilation is present in the rooms. Modern windows often cause a big build-up of water vapour in the air. Sometimes modern windows have trickle vents to counteract the built-in draught proofing round the frames. If your windows have trickle vents, they should be left open.

If you still have a mould or condensation problem

The above suggestions are all easy things to do which should cure, or at least improve, your mould or condensation problem. There are inevitably going to be situations where the problem refuses to go away though. If this is the case, the problem is almost certainly incorrect ventilation and you are going to have to call in a ventilation assessor. The assessor will then be able to provide you with the detailed advice and guidance to cure your particular humidity problem.


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