Condensation commonly appears as beads of moisture on non-absorbent surfaces. It may also occur on other surfaces, but may not be noticeable until the material is thoroughly wet. Blistering of paint, mould growth on walls and ceilings, water stains and a musty smell all indicate condensation problems.

The air around us contains water. The warmer the air, the more moisture it can hold. However, at any particular temperature the air can only hold a certain amount of water. When the air is holding the maximum amount, the temperature at which this saturation occurs is called the 'dewpoint'. Any cooling below this temperature causes condensation. The water leaves the air and forms beads or a film of water on any cooler surfaces such as windows, walls or ceilings.

In winter the outside air is usually so cold it cannot hold much moisture. When air enters a house it becomes warmer and can hold more moisture. Such moisture comes from cooking and washing, and also from the breathing and perspiration of the people in the house. A family of four can produce about 12 litres of water vapour each day in their home.

Water vapour is also produced by kerosene heaters and unflued gas fires. About 1 litre of water vapour is given off for every litre of kerosene, while unflued gas fires give off two litres of water vapour for every litre of natural gas burned. Such heaters should not be used where there are condensation problems.

In homes of older design and construction most of this moisture escapes from the interior through chimneys and air vents. In more modern homes where natural ventilation is reduced and there is more heating in winter, the air contains more moisture.

Ventilation is an essential safeguard against condensation from this cause but may lead to increased heating costs. The increase will be least where good thermal insulation is installed and ventilation is not excessive.