Condensation is dampness caused by water in the atmosphere. It is often first indicated by the presence of mould growth, which requires pure water for its growth and condensed water is pure.

In the home, excessive moisture builds in air as a result of daily activities such as washing, drying clothes, showers, cooking and simply humans breathing! When this moist air finds a cold surface with a temperature at dew point or below, the moisture condenses to leave the surface damp. Condensation often appears at low levels on wall surfaces where the wall is cooler, often where there is also limited air movement to remove saturated air. The appearance of this moisture profile is often mistaken for rising damp.

Condensation is a problem because if left untreated it can lead to mould growth on walls, ceilings, clothes and furnishings, and can also lead to rotting of woodwork.

Important numbers to consider when assessing condensation are the relative humidity of atmosphere (RH) and the dew point. Relative humidity is the amount of moisture held in air, expressed as a percentage of the saturation point of that air. Dew point is the temperature at which moisture in air condenses to form dew.

Dew point is important to understand in relation to condensation. The greater the amount of moisture in air, the higher the dew point. Therefore, where lots of moisture in being released into the atmosphere, from power showers, boiling kettles, etc, the easier it is for dew point to be reached.

As an example, if relative humidity in a room is 50% (a fairly ‘normal’ figure) and room temperature is at 21 degrees Celsius, the dew point is as high as 10 degrees Celsius. When there is lots of human activity causing increased RH in a building, it is more likely that the dew point will be reached.

Where condensation is occurring on the lower parts of walls, there can also develop a cycle of condensation in that area: Air coming into contact with the cold surface reaches dew point and deposits water. This cold air falls to the floor and pushes up warm air into its place. This warm air reaches dew point and deposits moisture, and so the cycle continues. The atmosphere in the room is becoming more dry as that area of wall acts as a dehumidifier, however this is creating dampness on the wall.

This process of wetting the wall will often be occurring while inhabitants of the room are finding it a warm and pleasant environment.

Why is condensation now such a common cause of dampness?

In times gone by, buildings were more draughty. Open fires and gaps around windows and doors meant that moisture from daily causes could exit the building and be replaced with new, drier air. However, improved heat (and therefore air) retention in properties, by for example closing chimneys and installing airtight UPVC windows and doors, has lead to increased likelihood of high humidity levels inside. The following are some examples of causes:

Cold bridging: This occurs where local cold areas, on walls that are otherwise above dew point, lead to areas of condensation. An example is the surface inside a cold concrete lintel.

Roof spaces: With limited ability to escape out of windows and doors within the living space, water vapour often finds its way higher up in a property and into the roof space. The best way to deal with this is to ensure good air ventilation within the roof space.

Sources of water vapour: Kettle, cooking, clothes drying on radiators, long, hot showers, breathing and sweating of humans!

Recommended actions for householders:

• Prevent moisture spread throughout a property: For example closing kitchen/bathroom doors when cooking/washing. Open windows in these rooms when in use, for rapid extraction of water vapour.
• Reduce moisture content: Provide ventilation to rooms so that water vapour can escape. Leave fans on a timer so that ventilation continues for some time. Trickle vents can be retro-fitted to windows and doors if required. Open windows in rooms where washing is left to dry. If using airing cupboards, try to provide some sort of ventilation.
• Provide some heating: This will increase the dew point of the building materials, by keeping them slightly warm for most of the time (rather than with blasts of strong occasional heating). So keep heating on at low level to maintain an ambient temperature of 10 degrees Celsius.
• New buildings: we note that new buildings often take a long time to dry out fully. In the first winter a new property may therefore require a higher level of heating and ventilation than in following winters.

This is not an exhaustive study of condensation in buildings. For further information please contact our helpline on 0800 078 6999 or email help@dehumidifier-rental.co.uk

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