Condensation in buildings
Air will generally include moisture in the form of water vapour.
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.
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. This interstitial condensation, when trapped within the structure by impervious materials is often erroneously referred to as Rising Damp.
Condensation affects the performance of buildings, causing problems such as:
- Mould growth which is 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.
- 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.
- 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, 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 of the building penetrate through its insulated fabric, for example around glazing, or where the structure penetrates the building envelope, such as at balconies.
- 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.
Some uses of buildings generate high levels of moisture and so specialist techniques may be necessary to prevent or mitigate the occurrence of condensation.
It is important that any systems introduced to limit condensation are properly installed and maintained to ensure continued optimal operation.
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.
 Related articles on Designing Buildings Wiki
- Approved Document F.
- Cold bridge.
- Designing out unintended consequences when applying solid wall insulation FB 79.
- Dew point.
- Dry-bulb temperature.
- Insulation specification.
- Interstitial condensation.
- Mould growth.
- Penetrating damp.
- Psychometric chart.
- Rising damp.
- Sling psychrometer.
- Solid wall insulation.
- Thermal comfort.
- Thermal indices.
- Treating brickwork with sealant or water repellent.
- Wet-bulb temperature.
 External references
Featured articles and news
We've analysed 6 million pieces of data to reveal that the knowledge framework underpinning the construction industry is no longer fit for purpose.
Read about the history behind one of California's most iconic buildings, the Griffith Observatory.
ICE examine just how close we are to providing subsidy-free low carbon electricity.
Have a look at MAD Architects' design proposal for renovating Montparnasse Tower into a concave mirror.
This article examines the legal issues behind off-site goods and materials.
Read about how technology is changing the real estate industry.
BRE Global introduce the first registration scheme for Suitably Qualified Security Specialists.
An introductory article to the different types of building foundations.
This unique Brutalist-era car park just off Oxford Street is soon to be demolished.
How to utilise technology in construction projects and what benefits will it bring?
Have a look at Thomas Heatherwick's new building, one he calls 'the tubiest in the world'.
Artificial intelligence will have a significant impact on the built environment, according to a new survey by ICE.