Dew point
Air will generally include moisture in the form of water vapour.
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.
RH = (actual water vapour density / saturation water vapour density) x 100
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.
Surface condensation occurs where water condenses on the exposed internal surfaces of a building, such as ‘cold’ windows.
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.
- Mildew.
- Staining.
- 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.
[edit] Related articles on Designing Buildings
- Cavity wall.
- Cold bridge.
- Condensation.
- Dehumidification.
- Deliquescence point.
- Diagnosing the causes of dampness (GR 5 revised).
- Dry-bulb temperature.
- Humidity.
- Insulation specification.
- Interstitial condensation.
- Moisture.
- Psychometric chart.
- Saturation.
- Sling psychrometer.
- Solid wall insulation.
- Thermal comfort.
- Thermal indices.
- Temperature.
- Understanding dampness.
- Water vapour.
- Wet-bulb temperature.
Featured articles and news
A transformative shift in the design, construction and management of built assets.
Apprenticeship announcement by the Prime Minister
Welcomed but with call for more actionable detail.
Heat pump announcements, what homeowners need to know
An 'ultimate guide to heat pumps' from a heating company.
Construction contract awards reach 7.1bn in February
Their highest level in seven months.
The journey to sustainability in heritage
Research is the key to better understanding.
Heritage approaches to adaptation, mitigation and loss.
Bridging the gap between policy, finance and installation.
Development on brownfield land
Definition, background, policy and the latest consultation.
With the Design Framework for Building Services.
Retrofit of Buildings, a CIOB Technical Publication
Pertinent technical issues, measures and the roles involved.
ECA joins HSE campaign to support mental health
Working Minds’ five simple steps based on risk assessment.
Mental health in the construction industry
Mental health issues in brief with related articles.
Transitional arrangements, Building Control and the BSR.
For pre-October buildings with substantial progress by April.
Why quality counts in domestic ventilation systems
From products, to systems to the installation.
Empowering the Future with CIOB Academy
Lifelong learning, upscaling, and reskilling for the built environment.