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The latest Crittall windows can achieve a U-value - a measure of their insulating performance - as low as 0.8, which is better than standard double-glazed windows.

Any building material you can think of will have a U-value, a measure of how easily heat passes through the material - the lower the number, the better.

Though still to be decided, incoming regulations in England could demand windows have a U-value of 0.8 or below.

Pilkington has a glass product called energiKare that can achieve a U-value of 0.9 in a double glazed centre pane, for example.

"It's thin and at the same time it has a very good, low U-value," she explains.