The Green Consideration - Energy Efficient Windows

“Windows Can Play Their Part”

Thermal properties of a standard window

The standard window!
Unfortunately in real life there is no such thing as a standard window, however from a point of view of legislation it is necessary to a standardize for calculation purposes, and to set a level playing field for industry throughout Europe.

The standard size for windows is 1230mm x 1480mm.

The thermal performance of that window is calculated using specialised software in accordance with a European standard EN ISO 10077 for the window frame and EN410 for the glass, and is measured in w/m2K (Watts / Metre Squared Kelvin), or the rate of heat loss per meter square of product per 1 degree difference between the external temperature and the internal temperature.

The minimum legal performance at present for the United Kingdom is 2.0 w/m2K

This minimum performance is a huge improvement on the average tradition single glazed window which has an average U-value of 5.8 w/m2K.

Consider the scenario of all the sash windows in the UK, estimated by English Heritage as some 44 million, were to change their windows from the 5.8 w/m²K to 2.0 w/m²K.

This would save each year 378 thousand tonnes of CO² being released into the atmosphere every year.

However this is only part of story.

It is a well known fact that windows lose heat through draughts, but not so obvious that windows gain heat, its called ‘Solar Heat Gain’ (G).

Many countries have adopted an Window Energy Rating which takes account of this heat gain in favour of the U-value.

The British Fenestration Ratings Council is the organisation who is pioneering in the UK such a calculation method, as detailed below..

Sponsored by: