Condensation occurs when surface temperatures are reduced as air passes across them. As the air gets cooler, the relative humidity rises and the water vapour turns into moisture. The air passing over the surface cannot hold onto the moisture and this becomes the water droplets that we then see. This happens in homes across the country when the temperature inside drops, particularly at night, and the moisture in our homes migrates to cold surfaces such as walls and windows.
You may not think there is much moisture in your home, but with four people in a property, each one will contribute in the region of four pints a day thanks to showers, breathing, boiling kettles and cooking. This adds up to more than 100 pints a week, and such a huge volume of moisture has to go somewhere. When combined with other domestic pollutants such as dust, tobacco, exhaust gases and chemical emissions it can create a serious health hazard. In the past, much of this would have escaped, but with better insulated homes, it now has nowhere to go, leaving stale humid air trapped and creating condensation and mould.