Time to Embrace Energy Ratings

Developed by the British Fenestration Rating Council (BFRC), an independent government-backed body, the system could eventually replace the traditional approach based solely on U-values. Rick Wilberforce, Market Development Manager for Pilkington Building Products – UK, explains why the new assessment method is a welcome development.

‘The concept of Window Energy Ratings has been developed on the simple premise that the traditional method of measuring the performance of a window, namely U-values, is an incomplete reflection of the impact windows have on the energy performance of a dwelling. U-values simply measure how much energy a window loses by conduction, they do not take into account heat lost by air infiltration or how much energy a window can gain through sunshine. Therefore, U values do nothing to reward or encourage the design of windows that create an optimum balance between these factors; hence the need for a comprehensive Window Energy Rating system.

‘The BFRC Rating combines the three factors which affect a window’s energy performance: conduction heat loss (U value), solar heat gain (g) and heat loss through air infiltration (L) – into a single performance number. These factors are linked by an equation, which produces the BFRC Rating:

BFRC Rating = (218.6 x g) – 68.5(U value + L)

‘The unit of the rating is kilowatt-hours per square metre per year. BFRC developed the equation so that, in a typical UK house, the value of the rating represents the net useful energy flow across the window throughout the year. If the rating is positive, it means the window is likely to be a net contributor of energy over the year.

‘The solar heat gain (g value) used in the equation is for the whole window, including the frame, not just the glass. Therefore a window with slim frames will have a higher g value than one with wider frame sections. Similarly, the U value used in the equation is that for the whole window. The value of L is the air leakage rate under average conditions.

‘For consumer purposes, the BFRC Rating value can be converted to a rating on an A to G scale (see illustration, left). This is the same scale that is used to rate the energy performance of other domestic goods, such as fridges, washing machines and light bulbs. The public is already familiar with the type of rating and label, and instantly understand that ‘A rated goods are energy efficient and G rated ones are not.’

‘The entire process takes place under the auspices of the BFRC, which has a network of approved consultants and companies, authorised to assess a window and calculate its rating. These are known as ‘Certified Simulators’. The Simulator then submits all the information to the BFRC, who issue the certificate to show a window’s BFRC Rating value and its position on the A to G scale.

‘Window Energy Ratings should be warmly welcomed and swiftly adopted.’