Rule 6: Condensation

Condensation problem

All properties have codnnsation, but most properties have sufficient ventilation to dry out condensation before it causes a problem.

Condensation results from excess vapour condensing of a cold surface, at or below the dew point.


There are three parts to the equation;

  1. humidity,
  2. temperature and
  3. ventilation,

Each one can cause and problem or resolve it.

Common problem

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.

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.

Most vulnerable areas

  1. cold water mains pipes,
  2. around edge of top floor rooms (by poorly insulated eaves),
  3. cold and poorly ventilated roof and sub-floor voids,
  4. cold spots such as north facing or shaded walls,
  5. near any embedded metal object (such as old gas lighting piping).
  6. external metal downpipes and supporting brackets,
  7. on and below windows,
  8. by openings, especially the cold section of wall by a door or window
  9. anywhere near a poorly ventilated bathroom, kitchen or laundry room,
  10. by double glazing, often with cold metal frames or without ventilation (many occupier don’t know how to open ajar on the safety lock).

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.


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.

Passive ventilation

A common mistake is to increase ventilation into a building. This is often 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. If the temperature is too warm to condense water vapour and the air outside it more than cold enough to condense vapour, then within the wall or building there exist a dew line. Vapour absorbed naturally by the building material will condense within the building material. This is called insterstilitial condensation. It is rarely visible and rarely causes a problem.

Cold metal embedded objects

Interstitial condensation is most noticeable when a building is composed of material with varying thermal properties, such as a cold wrought iron downpipes attached to walls or metal joist (RSJ). Water is often seen forming on a cold winter’s night below the steal beam of a roof extension.

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