How to Solve Condensation

Condensation is a problem in both period properties and newer homes, but what causes it and what can be done to prevent it?

What is Condensation?

The air – both that inside our homes and that outside them – contains moisture. The temperature of the air determines how much moisture it can hold, and warm air contains more moisture than cold air. When warm, moist air comes into contact with either a surface or air that is colder than it is, the warm air is unable to retain the same amount of moisture as it did and the water is released either into the cold air or onto the colder surface, causing condensation to form, quickly followed by mould.

What are the Causes?

Through our day-to-day activities – cooking, washing and drying clothes, heating and even breathing – we produce water vapour. Air can only hold so much moisture in the form of an invisible vapour, no matter what temperature it is. When the air contains more moisture than it can hold, it reaches ‘saturation point’ and when this is reached, the moisture turns back into water and condensation occurs. The temperature reached at saturation point is called the ‘dew point’. When this happens, the air has a relative humidity of 100%. The air in the majority of homes tends to have 50-70% relative humidity. Of course, none of us can realistically stop doing any of these things and if our homes are in good condition, with sufficient ventilation, we should not have to. Problems occur when structural defects in a building mean the moisture content has become too high; when old houses have no damp-proof course (DPC); and when there is inadequate ventilation in the home.

Period homes often have no DPC, which means moisture from the soil beneath the house rises up into ground floor rooms, whilst other homes suffer from bridged DPCs or damaged guttering. There are several types of condensation. Cold-bridge condensation occurs when warm, moisture-heavy air comes into contact with surfaces at or below its dew point. This occurs at the base of external walls – where it is often mistaken for rising damp – on windows, where it may cause cills to rot, and on the underside of the roof.

Warm-front insulation occurs when warm, damp air gets into a cold house. This happens in the winter, when a ‘warm front’ from the Atlantic arrives, and is common in unoccupied houses.

Interstitial condensation happens when warm, moist air diffuses into a vapour-permeable material, such as fibrous insulation. If this material is warm on one side and cold on the other, the moisture will be deposited in liquid form within the material. This is a particular problem in heavily insulated or air-conditioned homes.

How to Treat it

There are three basic ways to control the problem of condensation, by looking at relative humidity, ventilation and insulation.

Controlling the relative humidity in your home is one way to tackle condensation. Through the use of extractor fans in kitchens and bathrooms, the moisture levels can be kept at an acceptable level. Shutting the doors to these rooms whilst the extractor fans work also helps.

When it comes to ventilation, up until the mid 20th century most houses had natural forms of ventilation, with draughty windows, open fireplaces and gappy floorboards — and no central heating.

“Ventilation is needed to remove pollutants, moisture, cooking odours and exhaled CO2,” says Neville Boon of ventilation specialist AllergyPlus (01926 612690; allergyplus.co.uk). “During the heating season, the rate of ventilation needs to be regulated to be sufficient for the avoidance of surface condensation.”

Trickle vents in windows work well, but a more sophisticated option is a heat-recovery ventilation unit. These systems replace the air in your home, taking the stale, damp air outside, passing it over a heat exchanger which recovers heat normally lost through trickle vents, then bringing fresh air back in via a separate grille, passing it back over the heat exchanger to be warmed and put back into the house. Systems are available from AllergyPlus, Villavent (villavent.co.uk) and EnviroVent. It is also possible to buy central extract systems which connect all of the wet areas in your home to a central fan before discharging the stale, moist air outside.

Finally, to insulation. If all the walls in your home were at a temperature above the dew point of the air inside, condensation would be less of a problem. However, this is not realistic. Solid external walls tend to be cold by their very nature, so adding insulation, internally or externally, will help keep condensation under control, as will adding insulation to cavity walls.