David M. Kinsey

Why condensation happens
Condensation is governed by the way water molecules behave in air. Individual water molecules are like magnetic springs. If the molecules join together, they form water droplets. When the molecules are warmed, they bounce around more and come apart from each other, so the droplets dissipate. Conversely, the cooler the water molecules are, the less they bounce around so the more they join together. It is possible for there to be so many water molecules crammed into a space, it doesn't matter how much they bounce around, they can't avoid hitting other magnetic molecules and sticking together. The air in your house always contains water. While it's in the form of individual molecules, you can't see it and it causes no problems. When the little springy magnetic molecules start sticking together to form water droplets, you get condensation.

Common causes of condensation
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 how much the water molecules bounce around and hence their ability to join together. If the surface cools the air, and does it enough, a lot of the magnetic water molecules join together to form droplets which settle on the surface as condensation. If the air contains a large number of water molecules all crammed together, they hardly need be slowed down at all before they start to join together. This means even the slightest brush against a slightly cooler surface will make the water molecules form droplets.

How to stop condensation
You need to adopt a two part strategy:

Raise the temperature of the areas/surfaces where the condensation occurs;
Reduce the amount of water present in the air of the house.

There are a variety of ways to raise the temperature, depending on where the 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 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 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 in the air typically involves very simple changes. Major sources of water 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 in the air. Sometimes modern windows have trickle vents to counteract the built-in draft proofing round the frames. If your windows have trickle vents, they should be left open.

If you still have a condensation problem
The above suggestions are all easy things to do which should cure, or at least improve, your condensation problem. There are inevitably going to be situations where the problem refuses to go away though. If this is the case, you are going to have to call in a surveyor with the expertise and equipment to find out why it is such a problem. The surveyor will then be able to provide you with the detailed advice and guidance to cure your particular condensation problem.

The Period House Pathologist & Residential Building Surveyor

Diagnosing and Solving Problem Conditions in Houses Built after 1500