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Energy Efficient Windows

Energy Efficient Windows

Energy efficiency ratings for windows were launched in March 2004 by the British Fenestration Rating Council (BFRC). The system uses a rating scale of A to G, similar to the European Energy Efficient Labelling Scheme. An A rating represents a high energy efficiency and a G rating represents the lowest energy efficiency rated by the system.

Under the system, both the window glass and frames are tested and assessed together as one complete component. An energy rating is then awarded to the assembly as a whole. Assessment criteria includes heat loss (U value), solar gain (g value) and air leakage (L value), which all contribute to the overall energy efficiency performance of a window assembly. The system takes into account heat radiated through the window glass (solar gain) from the sun during the day and heat radiated through the glass from within the building during the night (heat loss). As windows can contribute to the heat within a building via solar gain, the U value alone is no longer regarded as an accurate indicator of window energy efficiency performance.

Building regulations for new build houses use the government's SAP (Standard Assessment Procedure) calculation method to demonstrate compliance by assessing predicted energy use and CO2 emissions for the dwelling. The calculation takes into account the U values, g values and L values of the windows, with the U value capped at 2.2 W/m2K (watts per square metre per degree Kelvin). In line with building regulations, the windows of existing dwellings should be replaced with windows achieving an energy efficiency rating of E or above. Windows within new extensions should achieve a rating of D or above.

A number of energy efficient window assemblies manufactured from wood, aluminium and uPVC are currently on the market in a variety of styles ranging from traditional to contemporary. Most are double glazed units and may also be injected and sealed with argon gas between window panes to minimise heat loss from thermal bridging.

Energy windows rating