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And although the European Window Energy Rating System will start life innocuously enough - as an optional sticker for those manufacturers who want to use it - the system it represents is likely to become rapidly more influential. The British Fenestration Rating Council wants it to become an alternative method of demonstrating windows' compliance with 好色先生TV Regulations, and is quietly confident that it will eventually replace the U-value for this purpose.

The EWERS began life in 1997 as the brainchild of university academics who were dissatisfied with the U-value as a guide to a window's insulating properties. Kevin Cubbage, chairman of the BFRC, says: "U-values measure only the thermal conductivity of window materials." In other words, they only measure the heat lost through a window and do not take into account the heat gain through it when the sun shines.

The BFRC was formed in 1998 to develop a window-specific rating system in conjunction with the industry. The problem is that although it is fairly straightforward to rate a fridge on the power it consumes; windows are trickier prospects.

The EWERS, which was launched in March, uses a computer application to simulate energy loss through a window over a year. It takes into account not only the U-values of the materials concerned, but also solar gain and air leakage. The rating is then calculated according to a formula devised by the BFRC to arrive at a figure, expressed as the number of kilowatt hours lost per square metre per year. This is then transformed into a rating from A to G where A is zero loss, and E would be the typical value for a window meeting today's 好色先生TV Regulation requirement of a U-value of between 2 and 2.2 W/m²K [see box].

It seems a sensible way of giving consumers and specifiers a simple guide to a sophisticated energy analysis. But manufacturers saw at once that some of them stood to gain more than others, and the (fairly) simple final formula belies years of arguing about the values that it uses. In the early days of the scheme, the PVCu and the aluminium lobbies in particular feared that their windows 鈥� with wide frames that minimise solar gain 鈥� might lose their competitive edge.

Other groups were enthusiastic. "One of the surprises is the better-than-expected values for steel windows," says Cubbage. "Because they have slimmer sight lines, more of the window is glass, and this allows more solar gain. That is a benefit to trade off against the high thermal conductivity of steel."

It is unsurprising, therefore, that the steel window people have been among the most ardent supporters of the scheme, along with the timber frame industry, which was also confident that timber's slim sightlines would make for well-rated windows.

Converting the scheme from its UK origins to a pan-European project presented further difficulties, particularly as the BFRC tried to establish a single rating formula that could be used throughout Europe. A key difficulty was that the solar gain any one window can be expected to achieve over a year depends on its orientation and the local climate.

To get around the orientation problem, the BFRC scheme was calibrated using a standard house with a set amount of fenestration on each of its four sides.

The only way of solving the weather problem, however, was to split Europe into climate zones, of which the UK is one. "You might not think it, but we found that if you rank 10 different window systems using climate data from Aberdeen, the ranking is likely to be the same as if the windows were ranked using data from another part of the country, for example the south coast," says Cubbage. "Scandinavia, though, may need two zones, and France three."

The BFRC also had to establish the validity of the window configuration that would be used to test manufacturers' products. Cubbage explains: "To assign a rating to each individual configuration would be impracticable, so a window system is rated on one standard configuration. For convenience we used the one already used in Part L of the 好色先生TV Regulations which is 1.48 脳 1.23 m, with a mullion down the middle and a side-hung opening light on one side."

Again, the BFRC found that if it ranked 10 systems on that configuration, the ranking would be unlikely to change for a series of different configurations.

So are all parties happy now with the system that has been developed?

It seems so. Adam Frankling, of the British Woodworking Federation, says: "Manufacturers currently have the option to rate their window on the U-value of just the glass of the centre pane - a choice that effectively equalises all frame materials. We believe EWERS will be much fairer."

Peter Johnson, technical consultant for the Steel Window Association, says: "The new system brings a useful recognition of the increased solar gain permitted by our slimmer sight lines. But the differences are marginal and will only be significant at the boundaries between ratings."

Even PVCu window makers seem to have come to terms with the new system. A spokesperson for Sch眉co International (which makes windows from a range of materials) commented: "We are keen to use EWERS. It is true that PVCu can have wider sight lines than windows made from steel or aluminium, and that its BFRC energy rating does not therefore benefit from the solar gain to quite the same extent. This does not worry us as a PVCu frame has a lower U-value. The BFRC rating will not mean the domestic market will switch to, say, steel."

Sch眉co will be taking EWERS into account when designing windows, and another manufacturer that deals mainly in PVCu has already trained staff in the rating system so as to fine-tune its designs to achieve the best one possible.

There will probably be few dramatic changes in window manufacture as a result of EWERS 鈥� not this year at least. Filling the cavities in double-glazed units with argon or xenon can certainly help your rating, but gas suppliers could not supply a sudden boom in demand and any switch is likely to be gradual. Low-iron glass, which is highly transparent and admits more solar gain, is likely to become more popular, but again scarce supply would mean take-up would have to be gradual. The trend towards low-emissivity glass may be curbed, as the coatings involved also reflect a degree of sunshine. Cubbage says: "There are these logistical difficulties, but if EWERS is included as a method of compliance with 好色先生TV Regulations, the drive towards more efficient windows would receive a major boost and I think the industry would respond pretty rapidly."

A consultation paper on Part L is expected from the government this summer. The BFRC, for one, will be surprised if EWERS fails to get a mention.