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Last edited 20 Sep 2018
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Relative humidity (RH) is a measure of the water vapour density of air compared to the water vapour density for saturated air at the same temperature and pressure (that is, the maximum amount of moisture that air can “hold” at that temperature and pressure). It is expressed as a percentage.
When air cools, it is less able to “hold” moisture, that is, the saturation water vapour density falls, and so relative humidity rises. When the relative humidity reaches 100%, the air will be saturated. This is described as the ‘dew point’ temperature, or the ‘saturation temperature’. If the air continues to cool, moisture will begin to condense. Where this condensate forms on a surface, it can be described as ‘dew’, hence the term ‘dew point’.
Understanding this phenomena is important in the design and construction of new buildings, and in the assessment of existing buildings, as the formation of condensation can be damaging, can affect comfort and can be a hazard to health.
Interstitial condensation occurs when the dew point temperature is reached within the fabric of the building, either on the surfaces of components that make up the fabric, or sometimes within the components themselves.
Condensation can cause:
- Mould growth which is a cause of respiratory allergies.
- Corrosion and decay of the building fabric.
- Frost damage
- Poor performance of insulation.
- Damage to equipment.
- Slip hazards.
Software is available to help calculate dew points, particularly in relation to interstitial condensation, the position of which within the building fabric is crucial if potential problems are to be avoided.
 Related articles on Designing Buildings Wiki
- Cavity wall.
- Cold bridge.
- Diagnosing the causes of dampness (GR 5 revised).
- Dry-bulb temperature.
- Insulation specification.
- Interstitial condensation.
- Psychometric chart.
- Sling psychrometer.
- Solid wall insulation.
- Thermal comfort.
- Thermal indices.
- Understanding dampness.
- Water vapour.
- Wet-bulb temperature.
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