Condensation seems innocent enough but it is the start of a more serious problem.
Condensation Principles
What is condensation?
Air contains invisible water vapour. The higher the air temperature, the more water vapour it can hold. The lower the air temperature, the less water vapour it can hold.
Where warm air contacts a cold surface, it cools. When the air cools below a temperature known as the 'dew point', invisible water vapour condenses to visible water droplets on the cold surface.
The water that is formed is known as condensate and the process is called condensation. If more water vapour is present, further condensation occurs which may lead to a trickle of condensate. However, the process is reversible - if the surface is warmed above the dew point, the condensation will evaporate and may leave the surface dry.
Common examples of surface condensation include:
Condensation In Buildings
Condensation within a building can form as visible surface condensation or can form on surfaces within the building fabric, known as interstitial condensation.
In cold weather, interstitial condensation is caused when water vapour inside a building is able to move outward via diffusion through permeable building fabrics or air movement and reach a surface within the building cavity that is below the dew point. That surface may be smooth such as sheet metal, or fibrous, such as glass wool insulation.
A cold surface that condenses vapour absorbs the heat of vapourisation, raising its temperature slightly. Thus condensation can be most rapid on a metal frame, and less rapid on insulation material. But given time, both might condense a considerable amount of water.
Interstitial condensation can be far more damaging to the building than surface condensation. Interstitial condensation can go unnoticed and if the building fabric has not been designed to allow moisture to dry from within it can become trapped and compromise the durability of the building and the health of the occupants.
The factors that contribute to condensation in buildings are essentially one or more of the following:
Moisture levels within buildings are often higher than outdoors. The main cause of high indoor moisture levels is the generation of warm moist air by domestic activities. Heaviest loads are produced by:
All of these factors contribute to raising the indoor relative humidity (RH). An increase in RH increases the dew point temperature for the same air temperature. This increases the risk of condensation should the water vapour come into contact with a surface below the dew point.
Image by Julia Filirovska
