Well to put it simply, a U Value is the measurement of the rate of heat loss through a material. Which means that
in all aspects of home design one should strive for the lowest U Values possible because the lower the U Value, the less heat
will be needlessly escaping.
A single glazed window with standard glass will have a typical centre pane U Value of 5.6, while a double glazed window
with standard glass will have a U Value of 2.8.
The calculation of U Values can be quite complex as it is measured as the amount of heat loss through a 1 square metre
of material for every degree difference in temperature either side of the material. It is indicated in units of watts
per square metre per degree Kelvin or W/m2K. Kelvin is used as the scale of temperature difference, but this is numerically
equal to degrees celsius.
So for example 1 square metre of a standard single glazed window will transmit about 5.6 watts of energy for each degree
difference either side of the window, or a U Value of 5.6. A double glazed window will be significantly better with
a U Value of 2.8, i.e. only transmitting 2.8 watts of energy in similar conditions.
Building regulations now specifiy that U Values must be 2.0 w/m2K or lower. This means that any new window will
need to comply with these regulations.
Anyone looking to purchase new windows should always ensure they are getting Low emissivity glass in the glazing, complete
with argon gas filled cavities to comply.
(The above text is only to be used as a guideline and regulations may vary in different areas of the UK.)
