Condensation Risk Report:
Covering England and Wales

Condensation risk analysis checks whether moisture is likely to form on the surface or inside the layers of a wall, roof or floor. It helps designers reduce the risk of mould, damp, decay and long term building fabric damage before construction work is carried out.

You need a condensation risk analysis to prove that a proposed wall, roof or floor build up is not likely to trap moisture or create mould risk. It is especially important when adding insulation, changing materials or designing airtight, energy efficient buildings.

Interstitial condensation is moisture that forms inside the layers of a building element, such as within insulation, timber, masonry or roof build ups. It can be hidden for a long time and may cause mould, rot, corrosion or reduced thermal performance.

Surface condensation happens when warm moist air touches a cold internal surface and turns into water droplets. It is often seen on windows, walls, corners and ceilings, and can lead to black mould if the surface stays damp for long periods.

Surface condensation forms on visible internal surfaces. Interstitial condensation forms inside the construction layers of walls, roofs or floors. Surface condensation is usually easier to spot, while interstitial condensation can stay hidden until it causes damage.

Building Regulations require buildings to be protected from harmful moisture, including surface and interstitial condensation. A condensation risk analysis is a common way to demonstrate that the proposed construction has been checked and designed to manage moisture risk.

In England, Approved Document C covers site preparation and resistance to moisture, including protection from surface and interstitial condensation. Ventilation, insulation and fabric design also interact with other parts of the Building Regulations, especially Part F and Part L.

Condensation risk analysis is commonly informed by BS 5250, BS EN ISO 13788 and, for more complex cases, BS EN 15026. The right method depends on the construction type, moisture risk, material behaviour and level of technical assessment required.

BS 5250 is the UK code of practice for managing moisture in buildings. It gives guidance on reducing moisture risks, including condensation, mould growth and moisture related damage through good design, detailing, ventilation and construction practice.

The Glaser method is a simplified calculation method used to assess condensation risk caused by water vapour diffusion through building layers. It is useful for many standard constructions, but it has limits and may not be suitable for more complex moisture risks.