Humidity
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.
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. If the air continues to cool, moisture will begin to condense.
Humidity influences thermal comfort. The higher the relative humidity, the less heat a person is able to lose heat through the evaporation of moisture on the skin, and so the hotter they will feel. Conversely, air that is too dry can cause problems such as dry eyes, nose, ears and throat. Typically, a relative humidity of 40 to 60% is appropriate in many buildings.
Humidity also affects the performance of buildings, causing condensation, mould growth, mildew, staining, slip hazards, damage to equipment and the corrosion and decay of the building fabric as well as poor performance of insulation. Condensation can occur on surfaces, or can be interstitial condensation, occurring between the layers of the building envelope, typically as a result of air diffusing from the warm interior of the building to the cool exterior and reaching its dew point within the building fabric.
Humidity can be measured using a hygrometer. Typically these are electronic moisture detectors, or devices such as a sling psychrometer which measures dry-bulb temperature and wet-bulb temperature, allowing relative humidity to be calculated or read from charts.
Humidity can be controlled by limiting sources of moisture (including reverse condensation, where moisture evaporates from damp materials), increasing temperatures, humidification or dehumidification, and by ventilation. Condensation can be further avoided by increasing surface temperatures (such as by the inclusion of insulation or by improving glazing).
In particular, it is necessary to avoid cold bridges, situations in a building where there is a direct connection between the inside and outside through one or more elements that are more thermally conductive than the rest of the building envelope, resulting in lower localised temperatures.
Humidity and condensation in buildings is regulated by Approved Document C (Site preparation and resistance to contaminates and moisture) and Approved Document F (Ventilation) and further guidance is available in BS 5250 Code of practice for the control of condensation in buildings.
[edit] Related articles on Designing Buildings Wiki
- Air conditioning.
- Approved Document F.
- Cold bridge.
- Condensation.
- Damp proofing.
- Designing HVAC to resist harmful microorganisms.
- Dew point.
- Diagnosing the causes of dampness (GR 5 revised).
- Draught proofing.
- Dry-bulb temperature.
- HVAC.
- HVAC balancing.
- Interstitial condensation.
- Methodology for moisture investigations in traditional buildings.
- Moisture.
- Moisture content.
- Mould growth.
- Preventing overheating.
- Psychometric chart.
- Rising damp.
- Sling psychrometer.
- Thermal comfort.
- Thermal indices.
- Temperature.
- U-value.
- Water vapour.
- Wet-bulb temperature.
Featured articles and news
Conserving the Hilda Besse Building
Improving facilities, accessibility and overall appearance.
BSRIA IAQ topic guide published
Free download of TG 12/2021 available.
Electrotechincal mentors wanted
TESP works with The Youth Group to form skill sharing network.
OpenBuilt supply chain technology in development
Big tech collaborates on platform for the built environment.
Concerns raised over Future Buildings Standard
Letter signed by 21 organisations sent to MHCLG.
The infrastructure planning process
A look at the Government's strategic approach.
Steps to help reduce the spread of infection inside buildings.
Urban exploration and construction
This social media-centred hobby can be both dangerous and illegal.
Millwork wall treatment with a long and illustrious history.
Weekly workplace noise estimator tool
HSE introduces cumulative exposure calculator.
The Edwardians and their houses.
Cut off from civilian life for over 900 years.
Can net zero and levelling-up align?
Gaining green support from the carbon giants.
Receive the Designing Buildings newsletter
Click the button to subscribe.