It's all a lot of hot air

Resist double glazing salesmen, advises Jeff Howell. They can't save us from global warming

Are you, in these midwinter months, being bothered by double glazing salesmen? They are annoying bastards at the best of times. But they have become especially irritating since 1 April 2002, when they could start claiming that, as well as fighting global warming and stopping old people dying from hypothermia, they had the British government on their side.

April Fool's Day was an appropriate date to launch the mishmash of convoluted reasoning that forms the new Part L1 of the Building Regulations for England and Wales, and which decrees that henceforth all new windows shall be double-glazed. The new rules include much genuflecting to the Kyoto Protocol, which committed western governments (though not the US, obviously) to reducing waste heat. Or "carbon emissions", as we are now supposed to call it.

Other European countries have decided to crack down on carbon emissions from power stations, factories and motor vehicles, but the British government has decided that our primary source of carbon emissions is poor people heating their homes with one-bar electric fires, and that if they would all just replace their windows with new double glazing, then that should do the trick.

They reached this conclusion after much unselfish committee-stage work by the British glass industry, which helpfully suggested that a thermal insulation value of two watts per square metre per degree centigrade would be about right. And now that the new building regulations have been unveiled, it must be gratifying for the glass-makers to find that the only way to achieve this new insulation value is by using sealed double-glazed units made with their own special low-emissivity ("low-e") glass. And I am sure it is nothing more than a coincidence that April saw the price of low-e glass rise by 5 per cent, followed by a further 10 per cent hike in September.

Low-e glass is very clever stuff. It has a metallic coating that reflects approximately 25 per cent of escaping heat back into the room. The downside is that the metallic coating also reflects approximately 25 per cent of incoming light back outside, so that the new windows appear much darker than ordinary glass.

It is a testament to the enduring scientific ignorance of the British public that they are consistently surprised by this. After all, heat and light are but a few wavelengths apart on the electromagnetic spectrum, and if you reflect one, then you are sure as hell likely to reflect the other. But the salesman didn't mention it, so the punters are puzzled. It's all done with mirrors, you see.

When we say "double glazing", what we really mean is "replacement windows". Most replacement windows are sold over the phone by commission-paid salesmen, and are made from PVC-U, or uPVC as it used to be called. The "U" stands for "unplasticised", to distinguish it from plasticised PVC, which is what raincoats are made out of. But though PVC-U is not as bendy as raincoat material, it is still a bit bendy, which is why the window frames are so chunky looking. They have to be, to stop the things from flopping around when you open them.

So the darkness inside homes fitted with replacement PVC-U windows is also a function of at least one-quarter of the window area being taken up with the framing material itself. Beautiful 1930s houses with delicate timber fenestration are having their windows replaced with PVC-U. They look as though their front elevations have been redrawn using a blunt pencil.

And despite their chunkiness, PVC-U window frames still have to be reinforced internally with steel or aluminium alloy bars. Which creates a bit of a problem when it comes to reaching the aforementioned thermal insulation value. For, contrary to popular perception, PVC is not a particularly good insulator. Considerably less effective than wood, for example. And when you poke a bit of metal down inside it, it becomes even more likely to conduct heat out of the house; which is why owners of new double glazing are often surprised to find condensation forming on the plastic frames of their new windows.

But the chief complaint of double-glazed homeowners is not about insulation, it is about internal misting. Sealed glazed units are bound to mist up sooner or later, due to the irrevocable forces of nature. The sealing material that joins the two panes of glass has to be flexible, otherwise the glass would break, and physics dictates that a flexible seal will also be vapour-permeable. Sealed glazed units cope with the constant seepage of moist air by incorporating a desiccant within the perforated alloy spacer bars that run around the perimeter. Eventually, the desiccant will become saturated, and the double glazing will mist up. This is also something that the salesmen never mention.

The timescale before the glazing mists up depends on the quality of the installation. Dry-glazed vented units in German windows may last 30 years. Cheap back-street British units stuck in with putty have been known to fail after three months.

Double glazing won't save you any money in the long run. If you take into account making the product, installing it, disposing of the old windows . . . well, it's like nuclear power; it's a net consumer of money and energy. There used to be an advert on the telly, with dear old Ted Moult saying: "You only fit double glazing once, so fit the best - fit Everest." So how come Everest is now leafleting all its customers, telling them they might wish to "upgrade" to the new standards?

Jeff Howell is a bricklayer, and building columnist for the Sunday Telegraph