What causes condensation?

Day-to-day activities such as cooking, washing and drying clothes, heating and even breathing produce water vapour. Air can only hold so much moisture in the form of an invisible vapour, no matter what temperature it is.

When the air contains more moisture than it can hold, it reaches ‘saturation point’. At this point the moisture turns back into water and condensation occurs. The temperature reached at saturation point is called the ‘dew point’. This can be quickly followed by mould.

When this happens, the air has a relative humidity of 100%. The air in the majority of homes tends to have 50-70% relative humidity. Problems occur when:

  • structural defects in a building mean the moisture content has become too high
  • old houses have no damp-proof course (DPC)
  • there is inadequate ventilation in the home.

Period homes like many in Sheffield often have no DPC. This means moisture from the soil beneath the house rises up into ground floor rooms, whilst other homes suffer from bridged DPCs or damaged guttering.

Types of Condensation

Cold-bridge condensation

This occurs when warm, moisture-heavy air comes into contact with surfaces at or below its dew point. This occurs at the base of external walls, where it is often mistaken for rising damp, on windows, where it may cause windowsills to rot, and on the underside of the roof.

Warm-front condensation

This occurs when warm, damp air gets into a cold house. This happens in the winter, when a ‘warm front’ from the Atlantic arrives and is common in unoccupied houses.

Interstitial condensation

This happens when warm, moist air diffuses into a vapor-permeable material, such as fibrous insulation. If this material is warm on one side and cold on the other, the moisture will be deposited in liquid form within the material. This is a particular problem in heavily insulated or air-conditioned homes.

Why has condensation become such a common cause of damp?

If the water vapour in the air of a home can escape somewhere, condensation would never occur. When homes were more draughty and open fires common, damp air would mainly escape up the chimney. Houses are now significantly better insulated and as a result, the moisture has nowhere to go and causing the significant growth in condensation problems.

Mould growth

Mould growth is a typical consequence of condensation problems in a home. Moulds are often most severe in room corners and on external walls. This is mainly because insufficient ventilation creates pockets of stagnant air in such corners.