Last edited 29 Mar 2019

Water vapour


Water vapour is the phase of water (H2O) that takes the form of a gas. It is a component of the Earth’s hydrologic cycle and is abundant in the atmosphere.

Boiling liquid water can produce water vapour by a process of evaporation, while ice can produce water vapour via sublimation. The use of water vapour in the form of steam has been a fundamental part of energy production since the Industrial Revolution.

Air will generally include moisture in the form of water vapour. Absolute humidity is the mass of water vapour in a volume of air divided by the mass of dry air. When air cools, it is less able to 'hold' moisture, that is, the saturation water vapour density falls, and so the relative humidity rises. When the relative humidity reaches 100%, the air will be saturated. This is described as the dew point. If the air continues to cool, moisture will begin to condense.

For more information see: Humidity.

The presence of water vapour in a building can lead to problems with condensation and moisture which can result in mould growth, damp, staining, and so on.

Moisture in buildings can be reduced by removing or reducing the source of water (such as washing processes, leaking pipes, rising damp and so on) or by drainage, ventilation, increasing temperatures or dehumidification.

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