Window Energy Ratings

What standards do I need to prove for my building windows?

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Ratings & Values

Window Energy Ratings are not a requirement of any standard, they are an aid to sales and marketing. They are helpful in ensuring customers understand the thermal performance of the window they are purchasing and can be used to prove the rating standard required by Building Regulations.

You DO need to have a thermal performance for your units or windows and this can be shown by an official WER, but there are a number of other ways to "prove" this

Pilkington and Saint Gobain have their own calculaters on line which can be used to provide a U value for your units..

  • Building Regulations

    A change in Building Regulations (Document L1) in October 2010 required all window installations from that date to have  a minimum, window energy rating of C.  Which is equivalent to 1.6 W/m2K.


    Since then there have been various revisions to Document L and the current value for insulating glass units is 1.4 W/m2K  as of June 2023.

  • A) Spacer Bar Thermal Values

    Although a small contributor to a WER, over the past few years suppliers of spacer bars have been claiming certain values. Many of these values have been disputed and to resolve the problems an agreement was reached by the Glass & Glazing Federation (GGF) and the British Fenestration Rating Council (BFRC) whereby testing must be undertaken to ensure that thermal values are comparable.


    A spacer bar manufacturer now has to submit data in the form of a Bundesverband Flachglas (BF) data sheet. This now means that all values are comparable and have been subjected to exactly the same tests.


    Whilst the thermal performance usually relates to the window,  you can get the U value for your units from the on line calculators for Pilkington and Saint Gobain.

  • B) Sealant depth.

    Sealant depth is critical to the performance of a double glazed unit.  The manufacturers guidelines should be followed and the units produced should  be representative of the System Description used for the Part 2 and 3 testing.