Condensation is a term used to describe the effects of warm moisture laden air that becomes cooler and ‘condenses’ when in contact with cold surfaces.

Warm air has the capability to hold more water in the form of water vapour than cold air. The amount of moisture in the air is described as relative humidity. Saturated air is said to have 100% relative humidity. The amount of moisture in the air is expressed as a percentage against saturation at the same air temperature is described as relative humidity. As air cools the relative humidity rises. As a general guide with every 10C decrease in air temperature, the relative humidity doubles.

Condensation generally occurs during the winter months when the external air temperature is cold. The external walls and windows are cold and by contrast the internal air temperature is warmer. The warm air takes up moisture and then cools down when in contact with the cold window frames, glass and walls. As the air temperature cools rapidly the relative humidity reaches saturation level, the dew point, and the air is unable to hold the moisture, condensation occurs and water droplets form. At 75% relative humidity block spot mould (mildew) will grow.

Condensation will manifest itself in different ways. On glass and window cills, water droplets appear and water will pool on the cills. On wall plaster mildew will form in the corners and in areas where the air stagnates. On clothes and shoes the mildew may be green and accompanied by a very musty smell.

The main sources of moisture within domestic properties are paraffin and gas heaters, cooking, washing and drying clothes and occupants. Excluding heating, it is estimated that a family of 4 or 5 will generate in excess of 14 litres of water per day. In addition an unventilated gas fire or paraffin heater will add another 5 litres of water. The water vapour must either be absorbed within the house, extracted correctly or will condense causing problems.

In order to overcome condensation related problems it will be necessary to address the situation at source and at cure. Heating the inside air alone is expensive and unpractical. Insulating walls and windows is necessary along with adequate air extraction or dehumidification. Maintaining background heat is essential and maintaining heating systems at constant temperature rather than on-off is preferable. Kitchens and bathrooms should ideally be fitted with adjustable humidity controlled extractor fans and if the problem is severe or persistent then a positive pressure fan can help to forcibly remove the humid air.