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.
Typically this happens in buildings when warm, moist air comes into contact with cooler surfaces that are at or below the dew point (such as windows) and water condenses on those surfaces.
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.
Condensation affects the performance of buildings, causing problems such as:
- Mould growth, which can be a cause of respiratory allergies.
- Mildew.
- Staining.
- 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.
- Dehumidification.
- 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.
Some uses of buildings (such as swimming pools) can 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 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.
[edit] Related articles on Designing Buildings
- Approved Document F.
- Cold bridge.
- Condensation pipework.
- Damp.
- Damp proofing.
- Dehumidification.
- Designing out unintended consequences when applying solid wall insulation FB 79.
- Dew point.
- Diagnosing the causes of dampness (GR 5 revised).
- Dry-bulb temperature.
- Electrical resistance meters.
- Flashing.
- Humidification.
- Humidity.
- Hygrothermal.
- Interstitial condensation.
- Methodology for moisture investigations in traditional buildings.
- Moisture.
- Moisture content.
- Mould growth.
- Penetrating damp.
- Psychometric chart.
- Rising damp.
- Sling psychrometer.
- Spalling.
- Treating brickwork with sealant or water repellent.
- Water vapour.
- Wet-bulb temperature.
Featured articles and news
Boiler Upgrade Scheme and certifications consultation
Summary of government consultation which closes 11 June 2025.
Deputy editor of AT, Tim Fraser, discusses the newly formed society with its current chair, Chris Halligan MCIAT.
Barratt Lo-E passivhaus standard homes planned enmasse
With an initial 728 Lo-E homes across two sites and many more planned for the future.
Government urged to uphold Warm Homes commitment
ECA and industry bodies write to Government concerning its 13.2 billion Warm Homes manifesto commitment.
Places of Worship in Britain and Ireland, 1929-1990. Book review.
The emancipation of women in art.
CIOB Construction Manager of the Year 2025
Just one of the winners at the CIOB Awards 2025.
Call for independent National Grenfell oversight mechanism
MHCLG share findings of Building Safety Inquiry in letter to Secretary of State and Minister for Building Safety.
The Architectural Technology Awards
AT Awards now open for this the sixth decade of CIAT.
50th Golden anniversary ECA Edmundson awards
Deadline for submissions Friday 30 May 2025.
The benefits of precast, off-site foundation systems
Top ten benefits of this notable innovation.
Encouraging individuals to take action saving water at home, work, and in their communities.
Takes a community to support mental health and wellbeing
The why of becoming a Mental Health Instructor explained.
Mental health awareness week 13-18 May
The theme is communities, they can provide a sense of belonging, safety, support in hard times, and a sense purpose.
Mental health support on the rise but workers still struggling
CIOB Understanding Mental Health in the Built Environment 2025 shows.
Design and construction material libraries
Material, sample, product or detail libraries a key component of any architectural design practice.
Construction Products Reform Green Paper and Consultation
Still time to respond as consultation closes on 21 May 2025.
Resilient façade systems for smog reduction in Shanghai
A technical approach using computer simulation and analysis of solar radiation, wind patterns, and ventilation.
Comments
If anyone was uncertain about the causes of condensation in buildings I am sorry to say that this article is more likely to lead to more confusion, with some inaccuracies and poor emphasis. Unfortunately, I haven't the time to rewrite it at present.
I completely disagree. This is a very well researched article with links to a great deal of additional guidance.
If you have a specific issue, say what it is. Simply saying it is confusing but not explaining why is very unhelpful for other readers.
To explain it would require me to rewrite. Perhaps I will do that at some future stage. Thanks for your reply.
I spent a number of years researching thermodynamics so would be very interested to know what you think the problem is. Can you point me to any literature that sets out a different view of the subject?