The most common form of unwanted dampness in buildings is water from the air that forms as condensation.
The air in buildings can have a high level of relative humidity due to the activity of the occupants (e.g. cooking, drying clothes, breathing etc.). When this water laden air comes into contact with cold surfaces such as windows and cold walls it can condense, causing water to be deposited. The point at which the water held in the air changes from vapour to liquid is known as the dew point.
Condensation is often associated with poor heating and ventilation in buildings, but this simple view can be misleading. Condensation is chiefly a winter problem, as the external air temperature is low and external walls and windows are cold. The usual sequence of events is as follows:
- Cold air enters the building
- The air is warmed for the comfort of the occupants.
- The warm air takes up moisture.
- The warm, moist air comes into contact with cold surfaces, walls, windows, etc. and is cooled below its Dew Point.
- Condensation occurs as the excess moisture is released.
Walls in kitchens and bathrooms (where atmospheric moisture levels are usually highest), solid external walls, un-insulated solid floors and cold bridges such as concrete lintels set in cavity walls are commonly the areas in which condensation takes place.
Intermittent heating and cooling of the property can aggravate condensation problems, since it allows warm damp air to cool, reducing its capacity to hold water. Dew points are reduced allowing condensation to occur. When the air is reheated water is taken back into the air only to be deposited again when the air temperature drops again.
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