Whether it was in student digs or a tumble dryer-less first house, we’ve all dried our wet clothes on radiators, kept our windows sealed… and seen the inevitable mould have a field day.

When I pulled out a bookcase to clean my first flat, after selling it, there was a pink triangle of mould thigh-high that we’d been blissfully unaware we were breathing in for years.

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According to NHS Choices, mould can harm your health because it produces allergens, irritants and, sometimes, toxic substances.

“Inhaling or touching mould spores may cause an allergic reaction, such as sneezing, a runny nose, red eyes and skin rash. Moulds can also cause asthma attacks,” says the site.

So it’s worth dealing with the source of the problem – and with colder weather starting to hit, the experts are warning us to take action against condensation now.

“While weather conditions so far this autumn have been relatively clement, the arrival of Siberian swans to our shores this week has sparked speculation that we’re in for a cold snap,” says Berwyn Evans, UK product manager of Rentokil Property Care. “This is when condensation becomes a problem for homeowners.”

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What causes condensation?

The problem affects one in five homes in the UK, according to Evans, who explains: “Condensation occurs when warm air comes into contact with a cold surface. Warm air can hold a lot more water vapour than cold air – meaning that when it’s rapidly cooled by contact with a cold surface (a window, mirror or outside wall, for example), the vapour becomes water droplets, or condensation. 

“At this time of year it becomes particularly apparent, as homeowners dry their clothes indoors, and more people bathing and showering increases the relative humidity of the air.

“Homeowners also start to switch on the heating while keeping windows firmly shut against the cold. While sensible, this behaviour increases the humidity in properties. Condensation will form if the Relative Humidity (RH) rises above 70%.

“If a property is fully insulated and the windows are double glazed, the original draughts that once removed the excess moisture we produce during our everyday lives are locked in.”

Any activity in the home which creates steam – such as cooking and showering – will cause condensation to occur, unless there’s adequate ventilation.

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What problems does it cause?

“If condensation is allowed to settle on a surface for over six hours, then the perfect condition for mould is created,” says Evans.

But while most people will spot water trickling down windows and mould patches in corners, hidden condensation is common – and causes some surprising and damaging problems.

“Loft spaces can fall victim when warm air full of water vapour passes up through the ceiling, before cooling and condensing,” says Evans. “Sometimes, when double layers of insulation have been fitted in a loft, the natural eaves ventilation becomes blocked, making the loft damp.

“If this occurs, timbers in the loft can suffer mould damage. If one slope of the roof is in the shade and the other sunny, mould may only form on the colder side in the shade. The loft space can even become damp to the point where it may ‘rain’ inside! If water tanks are left uncovered and water evaporates, a similar effect may be seen.”

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How can I get rid of condensation?

First of all, by doing everything you can to ventilate your home naturally: opening windows and using fans when cooking and showering.

Avoid drying clothes on radiators, put lids on saucepans when cooking and make sure you heat your home consistently.

Evans adds: “The good news for homeowners is that there are a number of effective solutions, including Positive Input Ventilation (PIV) systems, which ventilate the home and diffuse and dilute damp air.”

But he warns property owners to “always seek the advice of property care specialists when evaluating the causes of their specific condensation issues, as every situation is different.”