If you are thinking about changing your windows you will need to think about U-values. A U-value measures of the rate at which heat escapes through a fabric, so the lower the figure, the better.  A U-value of zero means that no heat is escaping.  It is expressed as Watts per square metre (W/m²) but it is really only the number that matters.

A few basics. Building Regulations Part L(1A) state that windows must have a U-value of no more than 1.8W/m², compared to walls that must have no more than 0.3W/m². The very best triple glazed windows might achieve a U-value of 0.6W/m². So despite anything the double-glazing salesman might say, the windows will always be a weak point in your insulation.

But to put things in context, a single-glazed window will have a U-value of 5.0W/m², double-glazed will be between 2.2 and 1.2W/m² and triple-glazed 1.0 to 0.6W/m².

So replacing a single-glazed window with a reasonable quality double-glazed units will more than halve the heat loss. But bear in mind that only about 10 per cent of the total heat loss from the house will be through the windows. There is really no point in putting expensive double-glazed windows in uninsulated walls.

In a new build, installing double glazed windows is a must, but there is little point in triple-glazing unless you have super-insulated the rest of the house. Triple-glazing will cost about 20 per cent more than double-glazing and reduce the overall heat loss by about 5 per cent.

Upgrading single-glazed windows is always a good idea but you might want to think about secondary double-glazing. It is not as efficient as primary double-glazing – U-value will be about 2.8 to 3.0W/m² – but it will cost seriously less than replacement windows. And if your frames are in good shape why throw them away?

photo by elNico

About the author: Tim Pullen is eco-editor for Homebuilding & Renovating magazine, author of Simply Sustainable Homes and founder of sustainable property consultancy WeatherWorks.

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