Condensation is caused by humid air condensing on cold surfaces, condensation starts when a wall’s temperature falls below the “dew point”. The dew point increases as humidity rises. There is often a line within a wall where the temperature is below the dew point, this is called the dew point line. Walls are designed to absorb and evaporate moisture daily.

Cumulative effect

Damp is often cumulative. For example, condensation is more likely to form near a wall that is damp from penetrating rainwater. Likewise, rain will not evaporate as quickly if the wall surface is already humid through condensation. Furthermore, wet external walls are poor thermal insulators. North, North-Eastern and North-Western walls receive minimal warmth from the winter sun. Some damp only occurs infrequently, once every few years, resulting from persistent rain and wind. Damp detection depends on conditions during the survey.


Vapour condensing into water on cold surfaces is the most common form of dampness in the home. It is most prevalent on the lower surfaces of external ground floor walls. Warm moist air from a kitchen, bathroom, washing machine or drying clothes will condense rapidly when meeting a cold external wall, window or pipe. Add to this humid breath from human and pets.

Dew point

The dew point is the temperature that water starts to condense. Humid air from a warm moist kitchen readily condenses on the cooler surfaces of external walls. Typically, a surface only has to be 5°C lower than the ambient temperature for condensation to start to form.


Cold spots

A wall will be relatively cold at the point where both skins of a cavity wall meet.  This is most pronounced at the base of a ground floor wall, which is often more than 5°C cooler than the ambient temperature. The temperature differential can be much greater at night.

Glass and metal are good conductors of heat and therefore lose thermal energy much more rapidly than timber, brick or plaster. Condensation runs down cold windows and frames onto walls beneath them. Metal objects embedded in walls such as behind an electrical socket, cable or pipe can initiate condensation. Cold metal can cause condensation, even in summer.

Although condensation is inevitable, it can be manged with ventilation out at source, combined with sufficient heat, air circulation and regular wiping of wet surfaces.

Drying clothes

Ideally clothes should be dried outside, or with an externally vented clothes drier. Double glazed windows should have trickled vents kept open.

An alternative is to designate wet areas, then manage humidity in those wet areas, by wiping off surface moisture and opening windows often. Victorians used to tile their entrance halls, at the point where cold air meets warm humid air. Bathroom paints and tiles evaporate moisture readily and are easy to wipe down. Top tip; use an electrically heated bathroom mirror.

Positive air flow – cons

A common mistake is to increase ventilation into a building. This can be counterproductive as the outside air is likely to be cooler than the warm moist internal air, and will cause, rather than alleviate condensation. Positive flow ventilation systems do not necessarily reduce condensation.

Interstitial condensation

In the worst cases, condensation can form within a wall. This is known as interstitial condensation. We will not be able to identify interstitial condensation unless it visibly affects internal decoration.

Condensation & evaporation in constant equilibrium

Any temperature above the dew point causes more evaporation than condensation. Any temperature below the dew point cause greater condensation that evaporation. This is the principle of constant equilibrium.

Without airflow, evaporation increases the surface relative humidity, thereby increases the dew point.

So the only way to overcome condensation is through evaporation, airflow and extracting humid air at source.