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
Investors in People: CIOB achieves gold
Reflecting a commitment to employees and members.
Scratching beneath the surface; a guide to selection.
ECA 2024 Apprentice of the Year Award
Entries open for submission until May 31.
UK gov apprenticeship funding from April 2024
Brief summary the policy paper updated in March.
For the World Autism Awareness Month of April.
70+ experts appointed to public sector fire safety framework
The Fire Safety (FS2) Framework from LHC Procurement.
Project and programme management codes of practice
CIOB publications for built environment professionals.
The ECA Industry Awards 2024 now open !
Recognising the best in the electrotechnical industry.
Sustainable development concepts decade by decade.
The regenerative structural engineer
A call for design that will repair the natural world.
Buildings that mimic the restorative aspects found in nature.
CIAT publishes Principal Designer Competency Framework
For those considering applying for registration as a PD.
BSRIA Building Reg's guidance: The second staircase
An overview focusing on aspects which most affect the building services industry.
Design codes and pattern books
Harmonious proportions and golden sections.
Introducing or next Guest Editor Arun Baybars
Practising architect and design panel review member.
Quick summary by size, shape, test, material, use or bonding..