A subsidiary of the Glass and Glazing Federation introduced a ratings scheme to measure and certify the energy efficiency of windows. Therefore helping consumers to make a more informed choice of window products.
The Energy Efficient Window rating uses a traffic-light system to illustrate the level of efficiency, like that used for ‘white-goods’ such as fridges and washing machines. Rating levels range from the most efficient ‘A’ rating through to ‘G’ with the most energy efficient windows (those rated A to C) also carrying the Energy Saving Recommended logo issued by the Energy Saving Trust.
How a window gets rated
In order to obtain an Energy Efficient Window Rating a fabricator or installer must manufacture a window (frame, casement, glass and all) and send it to have it's energy saving properties measured and a rating awarded.
The energy rating is calculated using three key factors, which affect the windows energy performance:
Solar Gain
Solar Gain refers to the increase in temperature that results from sunlight. It is the most important influencing factor and is measured in g-Value. To obtain the best ratings the g-Value should be greater than 60% and the best performance is achieved by use of low iron glass and slim sight line profiles.
Thermal Transmittance
Thermal Transmittance (U-Value) is the rate of transfer of heat. In the case of windows, it refers to the transfer of heat through a one metre square window divided by the difference in temperature of both sides. Therefore the glazing unit, the spacer bar and the window profile are all included within the calculation.
To improve the U-Value of the window, the sealed double-glazed cavity may be filled with gas in place of air. As the density of the gas is greater than that of air it further reduces the conductive and convective heat transfers. Argon is commonly used for this purpose as it has excellent thermal performance is is also cost-effective. Other technique used to improve the u-Value includes the use Low-E glass, in which a microscopic low emissivity coating is applied to the surface of the glass to reflect heat back into the room.
Air Leakage
Out of the three key factors, Air Leakage contributes the least to the energy rating but never the less needs to be considered and is measured at 50 Pa pressure difference.
