The Window Energy Label uses a traffic-light system to illustrate the level of efficiency, similar to that used for ‘white-goods’ such as fridges and washing machines.
The Window Energy Label provides us with five key items of information, which together help consumers understand the energy efficiency of the window. Of these the most discussed is the ‘Rating Level’ as this provides an overall indication of the windows performance.
The following shows the skeleton of a Window Energy Label. Unfortunately as we do not pay a membership fee to the federation that introduced the scheme, we are not allowed to show the complete label, however we hope that the illustration offers some guidance.
Five key items explained
Rating Level
Window Rating Levels range from the most efficient ‘A— rating through to ‘G’ rating. The rating tells us about the product as an insulator, so ‘A’ rated windows are better insulators than ‘B’ rated windows, ‘B’ rated windows are better insulators than ‘C’ rated windows etc.
Windows that are C rated or better are included in the Energy Saving Trust scheme and achieve an Energy Saving Recommended mark, which offers you increased peace of mind and increased energy savings.
Energy Index
The Energy Index tells us how much energy the window will save or lose. ‘A’ rated will save energy and so have a positive Energy Index, while ‘B’ rated and below will loose energy and so have a negative one.
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-glazing 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.
Solar Factor
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.
Effective 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.
How the Rating Level is awarded
During testing windows are awarded a g-Value, u-Value, L50-Value figures, these relate to Solar Factor, Window Thermal Transmittance and Air Leakage respectively. The Energy Rating equation is then used to get a Energy Rating.
Energy Rating = 218.6 x g-Value - 68.5 x (u-Value + L50-Value)
| Energy Rating Level | Energy Rating |
|---|---|
| A | 0 or greater |
| B | -10 < 0 |
| C | -20 to < -10 |
| D | -30 to < -20 |
| E | -50 to < -30 |
| F | -70 to < -50 |
| G | -Less than -70 |
