The British Standards Institute (BSI) set up Kitemark Window Energy Ratings in 2010 in additional to the existing Window Energy Ratings Scheme previously established by the British Fenestration Rating Council (BFRC).

Under either scheme double glazing and triple glazing systems manufacturers and suppliers are able to apply to join if they can fulfil stringent criteria, to ensure that the products they supply and fit confirm to the current Building Regulations, and are given an accurate independent rating.

BSI Kitemark Window Energy Ratings is therefore a third party product certification scheme which results in the certification of the energy rating of a double glazing manufacturers windows. In order for any particular combination of window frame, system, polyamide strip, gaskets and glazed sealed unit to receive an energy rating the manufacturer must a) obtain simulations from a BSI accredited simulator and b) demonstrate their ability to manufacture the windows defined by the simulations by controlling the supply of components and fabrication processes. This is enforced by continuous independent assessment by a BSI assessor every 6 months.

BSI simulations define the potential thermal performance of the window, taking into account the U value of the materials used, the solar gain (g value) and air leakage of the window design.

Hazlemere Window Company Ltd were awarded their BSI Kitemark Licence in 2010 in respect of Window Energy Ratings. Their Kitemark Licence Number is KM 566333 and each Hazlemere Windows customer who have double glazing or triple glazing installed by Hazlemere receive a BSI Certificate for their particular combination of products stating its Window Energy Rating (A, B or C).