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Last edited 15 Sep 2020
Condensation in buildings
When air cools, it is less able to 'hold' moisture, that is, the saturation water vapour density falls, and so the 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.
Moisture can also form as interstitial condensation - occurring within the layers of the building fabric - typically as a result of air diffusing from the warm interior of a building to the cool exterior and reaching its dew point within the construction of the building itself. For more information see: Interstitial condensation.
- Mould growth, which can be a cause of respiratory allergies.
- Slip hazards.
- Damage to equipment.
- Corrosion and decay of the building fabric.
- Poor performance of insulation (see Insulation specification for more information).
Condensation can be controlled by:
- Limiting sources of moisture (including reverse condensation, where moisture evaporates from damp materials). For example, replacing flueless gas or oil heaters, providing ventilated spaces for drying clothes, cooking and so on.
- Increasing air temperatures.
- Natural or mechanical ventilation. This is particularly important in cold roofs, where unseen problems can build up, putting occupants in danger of structural collapse. See cold roof for more information.
- Increasing surface temperatures, such as by the inclusion of insulation or by improving glazing.
- Avoiding cold bridges. These are situations 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. Thermal bridges are common in older buildings, which may be poorly constructed, poorly insulated and with single skin construction and single glazing. In modern buildings, thermal bridging can occur because of poor design, or poor workmanship. This is common where elements penetrate through the insulated fabric of the building, for example around glazing, or where the structure penetrates the building envelope, such as at balconies. For more information see: Cold bridge.
- The introduction of vapour barriers (vapour control layers) which prevent moisture from diffusing through the building fabric to a point where temperatures might be low enough to reach dew point. For more information see: Vapour barrier.
Condensation in buildings is regulated by Part C of the building regulations, and guidance about how to deal with common situations is given in Approved Document C (Site preparation and resistance to contaminates and moisture) and Approved Document F (Ventilation). Further guidance is available in BS 5250 Code of practice for the control of condensation in buildings.
 Related articles on Designing Buildings Wiki
- Approved Document F.
- Cold bridge.
- Damp proofing.
- Designing out unintended consequences when applying solid wall insulation FB 79.
- Dew point.
- Diagnosing the causes of dampness (GR 5 revised).
- Dry-bulb temperature.
- Interstitial condensation.
- Methodology for moisture investigations in traditional buildings.
- Moisture content.
- Mould growth.
- Penetrating damp.
- Psychometric chart.
- Rising damp.
- Sling psychrometer.
- Treating brickwork with sealant or water repellent.
- Water vapour.
- Wet-bulb temperature.
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