U Values for secondary windows
U values or WERs?
In 2010 Part L of the Building Regulations was amended so that the measuring of energy performance of new windows (that is replacement double glazing and windows fitted into new properties) moved from the U Values system to the Window Energy Rating (WER) system.
However these Regulations do not apply to secondary glazing.
Unfortunately this means that it is difficult to compare the overall benefits of secondary windows compared to replacement windows.
The two systems differ considerably - the U Value system only measures the centre pane value whilst the WER system takes into account the entire windows performance including the U Values.
To help in comparing we have calculated the typical thermal performance of secondary glazing using the WER system.
If you install a secondary window with 6mm low emissivity glass over a standard single-glazed primary window you will typically achieve a WER Rating of B or C.
If you install a secondary window with a double glazed low emissivity unit you will typically achieve a WER rating of A.
*** to see the Historic Scotland summary report please click here
*** to see the Historic Scotland full report please click here