Consumers, specifiers and egislators are increasingly focusing on the energy efficiency of building products. To respond to the challenge of raised expectations, the British Fenestration Rating Council (BFRC), in conjunction with the UK glazing industry and European partners has designed a window rating system to meet this need for simple and accurate information. The BFRC provides third party certification for the total thermal performance of windows using a ‘fair, accurate and credible’ assessment scheme. Windows are rated using the familiar A to G scale on the basis of their total energy efficiency, where an A-rated window is more energy efficient than a G-rated window. Consumers and specifiers can quickly and easily choose the most suitable window for their needs, government inspectors and agencies can quickly and easily see if the window meets the legal requirements and energy agencies can see if the window meets their criteria for support. The Window Energy rating assesses the whole window energy performance and covers the frame material, the frame design, the glass type and all the other components that make up the window. The rating is carried out by computer simulation and gives a single number that can be used to compare the energy performance of a window simply and quickly.