condensation on window
It’s that time of year again when all Property Management teams receive numerous complaints from building occupiers about dampness, and possibly mould growth, affecting walls and windows. In the vast majority of cases the cause is not a building defect and is instead condensation, which is a natural process and can be difficult to ‘solve’ as it commonly requires behavioural changes rather than building modifications.


Condensation is a relatively modern problem. In the past, homes were not well heated and  had numerous draughts, and there were no showers, tumble dryers, dishwashers, steam irons etc. There was therefore less moisture vapour in the air, and when it did occur, it was able to escape through the gaps around windows and doors, up chimneys etc.

As buildings have been modernised and fitted with sealed windows, central heating and modern appliances, and openings such as fireplaces have been blocked off, the risk of condensation is greatly increased. In essence, condensation is a consequence of the understandable desire for increased comfort levels and energy efficiency in our homes.


condensation on windowIn order to occur, condensation requires warm moist air, a lack of ventilation and a cold surface. It follows that reducing the risk of condensation therefore means improving one or more of the following:

Water Vapour Production

Reducing water vapour production can be difficult to achieve in practice, however basic examples include using lids on saucepans and avoiding drying clothes on an airer or radiator.


  • An increase in ventilation is generally achievable, and likely to have the greatest impact. For example:
  • Open a window when a room is in use, and for 20 minutes afterwards.
  • Use extractor fans in bathrooms and kitchens, fitted with overrun timers.
  • Keep doors to bathrooms and kitchens closed to help prevent water vapour moving round the home.
  • If windows have trickle vents, keep them open. Consider having them retrofitted if not.
  • In particularly problematic environments there are more costly, but often very effective, mechanical solutions available, such as positive pressure ventilation systems.

 Surface Temperature

mould around bay windowThe key measure is to keep heating on continuously rather than having a stop-start cycle. There are two reasons for this:

1.     The surfaces on which condensation can occur are kept warm.

2.     Warm air holds more moisture. If the home is heated only when occupied, which generally corresponds to vapour-producing activities, and the air temperature is then allowed to fall, the air will no longer be able to hold the same volume of water and the vapour will condense more readily.

This is particularly important during very cold weather, when the air/surface temperature difference will be greatest.

For similar reasons, rooms with the heating turned off can be prone to condensation, even if unused. Keeping heating on, and doors closed, will help. Ventilate these rooms as well if possible.

Installing cavity wall insulation can help raise the internal temperature of outside walls, and therefore assist in preventing condensation. However it must be installed correctly or it is likely to make things worse rather than better, and it can be difficult (if not impossible) to remove once installed. Cavity wall insulation is very unlikely to ‘cure’ condensation on its own, and will have no effect on other condensation-prone surfaces such as glazing.

Reducing internal air temperatures on a continuous basis (i.e. setting heating thermostats lower) can also assist though may not be desirable for comfort.



mould in wall cornerLiquid water produced by condensation is relatively free of contaminants hence provides a good source of moisture for the mould spores, which are present naturally in the environment, to thrive. In contrast, water that has passed through a building component, for example from a leaking roof, is unlikely to provide suitable conditions.

As such mould growth, which is black in colour and found in corners around windows or behind curtains or furniture, is generally indicative of chronic condensation. The mould itself can be cleaned off with fungicidal wash, which is commonly available in DIY stores, however if no action is taken to control the condensation then it will return.



  • diagramNew homes, or homes that have recently been re-plastered, will be more prone to condensation as they will contain higher than average levels of moisture until they have fully dried out, which can take several months.
  • Avoid installing wardrobes (particularly fitted wardrobes) against outside walls – the combination of poor air circulation and a cold outside wall is likely to promote condensation inside the wardrobe, potentially causing damage to clothes etc.
  • Keep gaps between furniture items and between furniture and walls to allow air flow. In particular, keep beds away from external walls.
  • Make sure tumble drier vents and other outlets are properly installed and sealed.
  • Do not block up air bricks and other vents, as this can promote condensation within the fabric of the building which can lead to other issues.
  • If installing loft insulation leave a gap around the eaves in order to allow air to circulate through the roof space and prevent condensation forming on the roof timbers. Also check that extractor fans are properly connected to vents, and are not venting directly into the roof space.

Dealing with condensation is not easy. Only carrying out one or two of the above steps may not solve your problem. You need to do as many as possible every day, so that it becomes part of your habits and lifestyle.

water produced chartStriking the right balance between warmth and ventilation is important and can be very effective.  By opening windows or ventilating the property it may appear that you are losing some heat, but what you are actually doing is allowing warm moisture-laden air to escape and permitting cool dry air to enter the home. Dry cool air is actually cheaper to heat than warm moist air.

If you are in doubt as to the source of mould growth in your property, or you require further advice on the causes and treatment of condensation, our qualified and experienced team at Sheldon Bosley will be pleased to assist.