How to cope with condensation

Douglas Kent, technical and research director for the Society for the Protection of Ancient Buildings (SPAB), gives expert guidance and insights on the causes of condensation in old houses and how to avoid them

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Condensation is distinct from other forms of dampness, such as rain penetration and rising damp. It is water released when air is cooled to its dewpoint temperature and unable to carry so much moisture as vapour – the opposite process to evaporation.

This may occur indoors when warm, damp air comes into contact with colder building components. It may lead to mould and timber decay. However, a few simple measures can often successfully reinstate the delicate moisture balance in old houses to prevent condensation.

Condensation on window panes and mould caused by condensation on walls

Condensation has formed on windowpanes; Condensation-related mould on walls

What causes condensation?

Condensation can arise when more moisture is produced – often from cooking or washing. Insufficient ventilation is another cause, for example, due to double-glazing, blocking of flues and air bricks, or incorrect installation of roofing underlay.

  • Condensation occurs mostly in winter and might first be noticed when water droplets form on hard surfaces, or mould appears on absorbent finishes.
  • What is known as ‘interstitial’ condensation occurs within the building materials and elements. For example, if a wall is covered with an impermeable barrier or renders that doesn’t allow moisture to pass through the wall naturally.

Employ an independent chartered surveyor or consultant (not a contractor with a vested commercial interest) if more detailed investigation is required.

How to stop condensation

In domestic buildings, simple lifestyle changes that lower humidity and/or keep surface temperatures above dewpoint generally provide a more practical and less expensive long-term solution than dehumidifiers or whole-house ventilation systems.

Generate less moisture

  • Place lids on pans while cooking
  • Dry clothes outdoors
  • Close kitchen and bathroom doors when in use
  • Vent tumble driers to the outside if not of the condenser type

Improving ventilation

  • Open windows and avoid draughtproofing those in kitchens and bathrooms.
  • Ensure that lofts, floor voids and redundant chimneys are well ventilated and avoid foam treatments on the underside of roofs that can cause timber decay.

Increased heating can maintain surfaces above dewpoint, especially if run constantly at a low level rather than intermittently. Lagging cold pipes may prevent surface condensation, as can improved insulation levels.

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