Property: When it comes to damp, the poor are in the cold; DOCTOR ON THE HOUSE

CONDENSATION is the biggest cause of dampness in British homes. It can occur in any building if the conditions are right, but local authority housing officials will tell you it occurs most often in post-war houses and flats because these are built with "modern" materials and because they often house poor people, who can't afford to heat their homes properly.

Condensation happens when water vapour in the air meets cold surfaces and turns into liquid water. The dew on the grass is condensation, hence the term "dewpoint" for the temperature at which condensation forms. In nature dew is very useful, as it provides plants and insects with a regular source of water even when there is no rain. Sometimes the dewpoint temperature is below zero, in which case it is called frostpoint, and then the water vapour turns into ice.

To understand condensation think about the dew, or frost, on the grass. Some so-called building experts will tell you that the way to stop condensation is to increase ventilation, but you can't get much more ventilated than outside in a field, can you? If it can happen there, it can happen anywhere. The architects and surveyors who spout this nonsense have usually been taken in by the sales literature of firms selling humidistat-switched extractor fans; these hardly ever work properly, and even if they did, they could not stop condensation on their own.

No, the only way to stop dew forming on the grass would be to warm it, and the only way to stop condensation forming on your bathroom ceiling is to warm it, and this means spending money on heating. So it is hardly surprising that poor people suffer most from condensation in their homes. There are around 8 million households in the UK suffering from what is called "fuel poverty", which, according to the British Medical Journal, is a greater percentage than in Siberia. Of course, our wonderful free- market energy companies do their best to help by making the poor pay more; those of us with incomes and bank accounts get discounts for paying by direct debit, while poor people charging up their plastic prepayment keys pay the full rate. So naturally they will prefer the cheapest fuel, bottled gas - but every litre of this, when burned, produces two litres of water vapour which ends up as condensation running down their walls.

I wrote last week that insulation on its own will not make your home warmer, which is true. Insulation can lead to more efficient use of heating, but it has to be applied sensibly or it can do more harm than good. Cavity wall insulation, for example, can cause rain penetration, wall tie corrosion, and can also increase condensation on the cold areas which the injected foam doesn't reach. Foam insulation sprayed on to the undersides of roofing slates invariably leads to rot in the battens and rafters. So how much money is the application of these products going to save you in the long run?

The Building Research Establishment publishes an excellent book called Thermal Insulation - avoiding risks. Like most BRE publications it is full of excellent illustrations and sound advice - and like most BRE publications it is only sold in a few specialist outlets. You can get a copy from Construction Research Publications for pounds 16.50 (01923 664444).

q You can contact Jeff Howell at the Independent on Sunday or by e-mail: Jeff@doctoronthehouse.demon.co.uk