Condensation is but a clue to what lurks within

says Jeff Howell

 

The falling temperatures are being matched by a corresponding rise in the number of readers' letters complaining about condensation. No surprises there - condensation occurs when warm, moist air meets cold surfaces. And the colder it is outside, the colder the windows and external walls become, increasing the likelihood that moisture from within the house will condense out on them.

 

Many readers, however, still think of condensation as something that occurs only on windows. But the fact is that if it is occurring on the window panes, it must also be occurring on the walls, or, more likely, within the walls. It's just that it is clearly visible only on the windows, because the misting-up is highlighted.
Condensation within the depths of the walls can be predicted by measuring the relative humidity and air temperature inside the house. Typically at this time of year, in an occupied house with the central heating on, these will be around 50 per cent RH and 20C, and we can look up these figures in the tables and see that a dew point temperature of around 9C is expected. This means that any surface that falls to 9C or below will have condensation forming on it.

 

So if the outside temperature falls to 5C, we will certainly see condensation forming on the window glass, but it will also be forming within the depth of the wall - about two-thirds of the way through, in fact. This will be unseen, but if it occurs regularly, night after night, the moisture content of the wall will build up, lowering its thermal insulation value, and supporting black mould growth on the wallpaper. And of course, in those homes with a relative humidity higher than 50 per cent - which will be those where a lot of cooking, clothes-washing and showering is going on - the dew point temperature will be higher (maybe 12C), so condensation will occur more often and with greater severity. These are the homes with black mould in the corners of the bedrooms, and green mildew on leather shoes and handbags in the cupboards.

 

So what's to be done? Well, the first thing is to get rid of the moisture in the air as soon as it is created, and before it can spread around the house. This means keeping the bathroom door shut, with the window open or extractor fan running, so that the moist air goes straight outside. Kitchen doors should also be kept closed, and cooker hoods are the best way of getting rid of the water vapour created by cooking (provided they are vented to the outside, of course, and not simply set to recirculate air through a carbon filter). And keeping the heating on will help keep the walls above dew point temperature, and will dry them out once the condensation has occurred.

 

And then there are dehumidifiers. These are clever devices, which extract moisture from the air before it can cause condensation, and thus they maintain the thermal insulation value of the walls. They also take latent heat from the moisture they extract (as do refrigerators) and recycle it into the house. In terms of energy used, a dehumidifier is three times more effective at removing water vapour than an extractor fan. Every home should have one.

 

Ebac dehumidifiers (0800 587 5859) are claimed to be designed for the British climate, with prices from about £150. Sunday Telegraph readers who mention 'Ask-Jeff' will receive a free five-year extended warranty.

 

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