Thermal Performance

Thermal conductivity of glass

Thermal performance is judged by the heat loss through a substance. In our industry this is the glass and window frame as a ‘combined’ product. This heat loss, which is the thermal conductivity of the glass and window products, is known as the U-value. It is expressed: W/m2.K.  For a full explanation, please click here.

In our market there are two commonly quoted U-value measurements

Centre pane U-values

This is the measurement of energy conductivity through the middle of a pane of glass whether it is single glazed, double glazed or triple glazed, etc. It does not take into account anything at the edge of the glass such as the spacer bar or window frame. Centre pane U-values can only be influenced by things that are present at the centre of the glazing – e.g. low e coatings, cavity width, Argon/Krypton gas or further panes – as in triple glazing.

Window U-values

This is the measurement of energy conductivity through the window that is made up of glazing and frame,  here spacer bar plays an important role, as may sealant. This is why window U-values are improved using warm edge spacer bars but centre pane U-values are not.

The lower the U-value, the better insulation performance of the unit

The higher the U-value of glazing or windows, then the higher the energy conductivity through them.
This means that lower U-values mean better performance in terms of insulation. This is why our customers often want lower U-values as they are proof of better performing products.

Window Energy Ratings

Window Energy Ratings (WERs) are controlled by the British Fenestration Rating Council – BFRC. Originally established in 1999 with assistance from Government and the major fenestration trade associations, BFRC was part of a research project to develop a Window Energy Rating scheme for the UK. BFRC Ltd was established in 2006 to take over and further develop the activities of the original BFRC. BFRC Ltd is now part of the Glass and Glazing Federation – GGF. Read more here.


Information contained on this website is a general description of the products and  Glasscraft Decorative Ltd hereby disclaims any liability arising from the inaccuracy or omissions from the publication and all consequences of relying on it.