By Terry Bartholomew

Condensation is the change in physical state from gas to the liquid phase and it can also be defined as the change in the state of water vapour to liquid when in contact with a liquid or solid surface or cloud condensation nuclei within the atmosphere. See British Standard BS 5250:2002 for Code of Practice for Control Condensation in Buildings.

Condensation in dwellings frequently happens in warmer rooms and areas with poor air circulation and typically can be seen on window frames and glass in the form of moisture droplets or as darkened patches of dampness on other surfaces. In areas with a continuous high level of humidity, it can also give rise to mould growth on window frames and wall/ ceiling surfaces.

At any temperature, air retains an amount of water vapour but the amount it retains is dependent on temperature and therefore, the warmer the air, the more water vapour the air can hold but only up to the point where it becomes saturated (known as the Dew Point).

When such warm air comes in contact with a cold surface, typically a window or external door, or the external wall of a dwelling, the temperature of the air will reduce rapidly causing the air to lose water with the result that moisture vapour will condense on the cold surface.

Condensation can occur all year around given the right climatic conditions but is more commonly associated with colder months where moisture-laden air moves from a centre of high concentration (typically kitchens, utility rooms and shower/bathrooms) to a centre of low concentration (typically bedrooms, especially with external walls).

Whether air is dry or water-laden is referred to as relative humidity (RH) and long periods of high RH will increase the risk of mould formation.

There are a number of fairly simple measures that can be implemented to reduce the risk of condensation as a result of human activity within a household, all of which are designed to increase the general temperature, improve ventilation, decrease the production of water vapour or a combination of both.

  • Adjust central heating controls to leave the system on a low setting throughout the day in cold
    weather, rather than it switching on and off. Avoid large differences between thermostat
    maximum and minimum temperature settings.
  • Remove condensation moisture from surfaces as it forms.
  • Allow air to circulate by ensuring there is ventilation under doors (but taking care not to
    impact on fire integrity), leave ventilation gaps to cupboard doors, keep furniture away
    from walls to enable air to circulate behind.
  • Ensure external air vents are fitted and not blocked.
  • In bathrooms and showers, where high levels of humidity can occur quickly, open windows when in use and make sure mechanical ventilation is fitted and working, including an override run-on facility after the lighting is switched off.
  • Keep lids on pans when cooking and use a good quality kitchen extractor fan. If a recirculating fan is fitted, make sure the filters are clean and changed regularly.
  • Avoid drying clothes in the dwelling if possible but if this can’t be avoided, select a cooler area.
  • Make sure your tumble dryer is properly vented to the outside or that the reservoir is emptied regularly if self-condensing.
  • Install trickle vents to windows permanent vents to external walls.

If the above measures have been considered and the problem still persists, there are further
changes that can be considered albeit more disruptive and expensive including:

  • Install a heat exchanger which will remove moist air reuse the thermal energy generated.
  • Install double glazing or secondary glazing with ventilation in the frames.
  • Thermally line window reveals with a plaster backed insulation board.
  • Thermal lining of colder walls