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Condensation and Mould Growth

The increased incidence of condensation in today’s buildings is the direct result of changes in modern living conditions which have led to warmer and more insulated homes. These changes have created rooms which often have less ventilation and fewer air changes. The water vapour produced by normal living activities is no longer able to escape through chimneys or through window joints or other outlets. Condensation and mould growth are the visible signs that the air in a building is too wet.

Water vapour is created by normal everyday living in our buildings such as breathing, perspiration, washing, cooking, bathing, drying clothes and burning fuel such as gas. The average family produces 12 litres (5 gallons) of moisture every day. Think of it as a two and a half builders’ buckets full of water to understand the scale of the problem the house has to cope with.

This can be reduced by:

• keeping lids on pans when cooking, keeping doors closed / windows open
• keeping bathroom doors closed when in use and afterwards to stop the spread of steam and moisture (use extractor fan or ventilate well after use for at least an hour with the door closed and/or put the heater on afterwards to achieve at least 21°C)
• drying clothes outside or in a tumble dryer with an outside vent
• opening windows when ironing
• not using bottled gas room heaters as they produce a lot of water vapour when burning

Air contains a certain amount of water vapour and this varies depending on the air temperature. The warmer the air, the more water vapour it can hold. This is expressed as Relative Humidity.

Condensation occurs when the RH is too high for the air temperature. The excess water is dumped on the nearest colder surface such as glazing. For example, if you have an air temperature of 16°C and a RH of 70%, lowering the temperature to 11°C would produce a RH of 100% (saturation) and consequently water droplets (condensation) will appear.

Scientists have shown that for people to feel comfortable indoors and to stay healthy, RH should be in the range of 30-60%, and the temperature should range between 18°C to 25°C.

Too much water vapour and too high a humidity can cause condensation. This appears as misting or water droplets on cold surfaces such as windows, walls or tiled areas when the air is too humid and cannot hold any more of the moisture.

Condensation appears when the temperature in the room drops below dew point when the air has absorbed all the moisture it can hold (at that particular temperature and pressure) and cannot hold any more moisture.

If at this point the room was heated up to a higher temperature, the air would hold more water vapour and less condensation would appear.

Mould spores are always in the air and their growth occurs when they germinate on contact with damp surfaces through condensation or rain water penetration. The mould takes the appearance of small black, grey or green spots on walls, windows and other surfaces.

Mould is most commonly seen around windows reveals or external walls and at high level at external corners. If there is mould growth at low level, it is not rising damp. It is just that that part of the wall is at or below dew point.

Mould growth requires the RH to exceed 70% for some time before it will start developing.

• human activity (breathing, perspiration, cooking, drying clothes indoors, etc.)
• inadequately heated / ventilated rooms
• colder walls that are more exposed to the outer environment
• thin brick walls
• north facing walls
• trees or shrubs preventing the room from being heated by the sun
• a leak to the building
• a new-build still drying out

Condensation and mould growth can often be controlled by:

• increasing indoor temperature and keeping it stable;
• increasing the flow of warm air to avoid cold pockets;
• allowing adequate ventilation;
• deploying a dehumidifier / large silica-gel cushions that will help absorb moisture from the air
(a short-term solution as it will deal with the symptoms, not the cause);
• keeping your glass clean (avoiding any aggressive chemicals as they can damage your blinds when in contact);
• keeping your blinds clean at all times (dirt forms a suitable organic matter for bacteria to thrive and mould to grow on;
• keeping blinds retracted when you know condensation is forming (in serious cases, take your blinds down if you are worried about damage and resolve your condensation problem first).

Ventilation allows a normal escape route for mist air. As the air in the property circulates, it draws moist air to the outside through open windows, doors, trickle vents, extractor fans, airbricks and chimneys and is replaced by fresh air. If this exchange is poor or prevented, the air indoors becomes saturated and water vapour will condensate on the nearest surface when the temperature drops below dew point.

Avoid creating still air pockets and cold spots. It is good practice to leave windows open for about

an hour a day, just enough to encourage air circulation but avoid allowing rooms to get too cold as this would only add to the problem.

To allow fresh air to circulate:

• fit extractor fans to shower rooms, bathrooms and kitchens;
• open all windows wide until condensation disappears (usually in 30-60min) and leave a 5mm gap to carry on ventilating afterwards if you can;
• ensure that trickle vents are open in double glazed windows

Ventilating should be done regularly and effectively to improve air flow.

Warm air can hold more moisture than cold air so if the property is not heated adequately, it is more likely to suffer from condensation. Warm air cooling in the night will still result in condensation, especially on or around windows during cold weather. Most of this will evaporate as the heating is turned on again in the morning and windows are opened.

Walls store heat. The amount they store depends on how heavy the materials are, their insulation standards and the period for which it has been heated. As the external wall cools down, the heat is lost to the outside atmosphere. If the heat is not replaced quickly enough by the heating system, the walls will continue to cool down until they fall below the dew point temperature and the condensation will begin to occur. The formation of the condensation cools the walls even further, resulting in even more condensation.

To cure this problem, provide more heat until the walls are warmed to a temperature above the dew point. It will initially cost more to get the walls warmed up but when they are properly dry, the heating bills will reduce again.

Anti-microbial finishes inhibit microbe activity, their growth and reproduction and also provide protection against fungus and certain bacteria strains. Anti-microbial coatings should be applied to all external fabrics and also to internal fabrics used in high moisture areas such as kitchens and bathrooms and environments where high levels of hygiene are essential e.g. hospitals and residential homes, schools and nurseries.

Shading products should always be cleaned following manufacturer’s instructions to avoid damage.

No, blinds do not cause mould. Condensation that is the breeding ground for mould must have already been present in the room and its presence would over time encourage mould in places where humid air meets cold surface (often on and around windows).


We acknowledge the BBSA as the authors of this article. This Guidance Note has been produced with reference to advice form Bournemouth Borough Council and Plymouth City Council.

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