In the past most homes had one or two chimneys, which allowed up to four air changes per hour and doors and windows were generally less well fitting than they are today. This natural ventilation was the very process, which prevented condensation. So condensation is a relatively new phenomenon resulting mainly from changes in our lifestyle and our desire to reduce heating costs.

To cope with increased fuel costs came the trend to insulate, bringing about loft insulation, cavity wall insulation and double glazed units. However, it is also a fact that energy-efficient homes are more likely to suffer from condensation – because anything that keeps warm air in will also keep fresh air out, creating the ideal conditions for condensation to form.

Before we tackle the problem, we have to understand exactly what condensation is, then find the best way to deal with it.

Condensation is merely the air’s natural moisture content settling on cool surfaces. The amount of moisture in the air is called relative humidity. Mould and mildew will be encouraged to grow if the humidity level rises about 70%.

Astonishingly, the average family creates up to 20 pints of moisture every day, simply by washing, cooking and breathing. This moisture must go somewhere and be dealt with to avoid condensation.

What is Condensation?

Condensation is water vapour suspended in air. The most common sources of water vapour are cooking, drying clothes on radiators, washing up, in house plants, moisture in newly built properties and extensions and from the breath we exhale.

Where can condensation occur?

Due to the thermal currents within a house, condensation can occur in a number of places, usually at cold spots i.e. an unheated room or a conservatory with inadequate conservatory heating.

Condensation forming on the room side surface of a sealed unit indicates a high water vapour content present and that the temperature of the room side glass surface is inadequate.

Condensation within the airspace of the sealed unit indicates the unit has broken down. This causes the appearance of misted double glazing and is known as failed double glazing.

“I did not have condensation before my new windows were installed. There must be something wrong with them.”

This is a common assumption but, unfortunately, it is also incorrect. Windows cannot and will not produce any water. This ‘water’ is produced by our normal living activities. Therefore we, the householder have created the problem.

How do I know if I have condensation?

Condensation will take many forms, the most common being steaming windows and puddles of water on the window sills. In extreme cases, dark spots of mould will appear around the windows, wall coverings and mastic seals throughout the house.

If you have a non-condensing tumble dryer, make sure it is properly vented to the outside of your home. Remember, tumble dryers can create 16 pints of water vapour during one cycle.

How can double glazing help?

Sealed unit replacement windows act as an insulator which will reduce heat loss which under normal circumstances, would be conducted from the inside of the room to the outside. Please remember that sealed units act as an insulator and are not a source of heat, therefore all rooms should be adequately heated – especially conservatories. The likelihood of condensation forming on a warm surface is therefore reduced.

How do I reduce condensation?

Condensation can be controlled by providing natural ventilation to change air on a regular basis and by maintaining an even temperature. This is achieved through ventilation units which are controlled by humidistats, an airbrick, or by opening a window or roof vent. Please remember that the airbrick must be open to achieve good results. An effective way of controlling condensation would be to install a dehumidifier. This cost could be avoided if the problem is one of ventilation, by installing an airbrick thereby creating ventilation or controlling those causes of moisture, that result in a build up of moisture in the air.

Breathing. Two sleeping adults exude two pints of moisture in 8 hours, which is absorbed as water vapour into the atmosphere.

New property. The bricks, timber, concrete and other materials in an average 3 bedroom house absorb 1500 gallons of water. The same principle will apply to a conservatory base and the construction materials used. As with any new building work, please allow a period of drying out to ensure problems are not encountered in the future.


By acting as a heat barrier and providing an inner pane which is considerably warmer than the outer pane, the likelihood of condensation forming is reduced.


Condensation is the result of a build up of moisture caused by our normal lifestyle and the continual improvement and modernisation of our homes.

Replacement windows cannot produce condensation. Double glazing will act as an insulator if there is sufficient heat within the house in the beginning. Therefore it is wise to attempt to control the amount of water vapour displaced within the household and to provide controlled ventilation to dispel this moisture before a problem arises.

Bathrooms & Kitchens

Prevent water vapour finding its way into the other rooms of your house by closing the adjoining doors and leaving a window open after cooking or showering to allow a change of air. Extractor fans and cooker hoods work well for this purpose.

If you find dark mould spots forming, treat the affected areas immediately with a solution of household bleach or Milton fluid. This will kill the mould spores and prevent them from spreading to other areas. All new houses with improved insulation and replacement windows are likely to trap moisture build-up. This can be identified and dealt with, as long as all the rooms are heated throughout the house, and the air is changed on a regular basis.

Remember it is far easier to treat the cause than the effect.