- Project plans
- Project activities
- Legislation and standards
- Industry context
Last edited 20 Sep 2018
To help develop this article, click 'Edit this article' above
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.
Featured articles and news
RSHP's Merano wins RIBA accolade.
How to differentiate between partial possession and early use.
Ofwat proposes £12 billion additional investment and £50 bill reductions.
Avoiding 'winner's curse' and other useful info.
Developing test methods for video flame/smoke detectors
Waiting for a new deal ...but will funding materialise?
Our servers have reached another milestone. Why not write an article and be seen by our 6.5 million users.
RSHP celebrates competition win in Paris.
All about approved inspectors.
Whilst apparently confusing, German conservation is actually not that different.
The rise and fall of council housing. Book review.
Drivers of change in global heating markets.