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
The minimum legal performance at
present for the United Kingdom is
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
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 British Fenestration Ratings
Council is the organisation who is
pioneering in the UK such a calculation
method, as detailed below..