BM TRADA Certification was appointed an independent agency by the British Fenestration Rating Council (BFRC) last year for its Window Energy Rating Scheme. Demand from manufacturers has been growing rapidly, driven by the April 1 changes to Part L, Conservation of Fuel and Power Regulations for England and Wales, which have placed renewed emphasis on fuel conservation.

The BFRC scheme is called up in Part L1B of the new regulations and is also supported by the Energy Saving Trust. And although the government has scrapped plans for the home sales pack, sellers will still require an energy efficiency rating for their house. BFRC-rated windows will therefore have an edge, with homeowners looking to add value and help boost the energy rating of their homes.

Around two million windows are installed every year in the UK, in new build and as replacements. Double-glazed windows are a standard item in any housing specification here but, according to the Energy Saving Trust, most only meet the minimum legal standard – the total potential energy savings from choosing higher performance products are substantial.

While these savings are not as large as those achievable from, for example, cavity wall insulation, with home energy use responsible for 27% of UK CO2 emissions, it makes sense to choose an energy efficient product for new build or refurbishment.

A window's energy performance has traditionally been stated in terms of its U-value. However, manufacturers may quote the U-value of the frame, the sealed unit or the whole window, so the performance of different products may not be readily comparable. Whole window U-value calculations, while clearly more useful, are complex to determine because they must include the thermal performance of a number of components.

This was too complicated for specifiers wanting to compare different types of window. Working with a number of European partners, the BFRC has devised a simplified scheme which expresses the overall energy rating of a window as a single number, after taking heat losses and solar gains into account. The number achieved then slots into a rating band, with 'A' the top rating and 'G' the lowest, indicated on a consumer-friendly, multi-coloured label, as seen on fridges, freezers, washing machines and light bulbs.

Window installers must have suitable quality management procedures in place

Window manufacturers are saying that there is an increasing requirement for “Minimum C-rated” windows as part of the quote specifications from specifiers. This is ultimately raising the game of all compliant manufacturers and, as a result, we are seeing more and more highly rated products in the scheme.

To achieve a rating, a window installer must also have suitable quality management procedures in place to guarantee the performance of the installed product – and consistency in manufacture. To achieve 'C' or above, windows must be well designed, well manufactured and use quality components, including effective weather seals and thermally efficient glazing. It is important for the manufactured window to incorporate well-fitted, good quality weather seals to the window openings and around the glazing perimeter. The window profiles and associated reinforcements (where applicable) must be designed to provide an effective thermal barrier to the external elements.

Different combinations of frame style, frame material and glazing can also yield very different results when trying to compare energy efficiency and cost. With the BFRC rating, it is easy to compare different windows, even those made with different frame materials. Timber windows, especially for bespoke designs, are gaining ground in the UK and timber offers several thermal advantages over some of the alternative materials on the market.

The traditional approach to improving the energy efficiency of a window has been to increase the number of layers of glass and air. Double- or triple-glazed windows have insulating air or gas-filled spaces between each pane. Each layer of glass and the air spaces resists heat flow, but it is important to use an efficient glass type for each of the inner and outer panes to maximise the thermal properties of the overall window. Each cavity has to be supported by a spacer bar and sealed accordingly. The spacer bar also needs to provide an efficient thermal barrier to maintain a good energy rating.

The width of the air spaces between the panes is important, too: air spaces that are too wide have higher U-values and can allow too much heat transfer. Highly energy efficient windows are manufactured with inert gases (argon or krypton) between the panes, because these gases transfer less heat than air.

Different combinations of frame style, frame material and glazing can also yield very different results when trying to compare energy efficiency and cost

The requirements for the energy efficiency of windows and houses are covered by the UK Building Regulations. The specific requirements depend on the location of the building and specifiers should check requirements for their own area with their local building control officer.

Manufacturers can be assessed for any number of product specifications within the scope of their certification. Each manufacturer in the scheme is listed on the BM TRADA website (www.bmtrada.com) and each specification will be identified on the BFRC website (www.bfrc.org).

The fact that BM TRADA is an independent, UKAS-accredited third-party certification body ensures that under the BFRC scheme manufacturers are regularly audited for conformity and consistency by fully trained, customer-focused auditors, ensuring a consistent end product that lives up to its energy rating.