Window Energy Ratings Explained
Windows are rated according to the new "Window Energy Ratings" (WERs), a system which has been introduced to meet the challenge of climate change and to help lower the cost of energy bills and which has been endorsed by the Energy Savings Trust. This is a legal requirement for all new build windows from October 2010.
WERs are a sophisticated ranking system, judging not just the energy efficiency of the frame or glass, but of the
whole window. WERs measure a window,s energy performance, rating how much heat the window retains, how
much heat a window gains through capturing sunlight and how little air is lost through the window. The objective of the Thermal Rating is to allow a relative comparison of the overall thermal efficiency of different window systems, comparing how the windows perform under standard conditions. The use of a window system with a good Thermal Rating does not guarantee that a home will be easy to heat during winter, and may lead to issues with solar overheating in summer unless a proper ventilation strategy is adopted.
Things to consider:
The better the U-value, the less heat is lost from inside of the property to the outside. The lower the U-Value
number the less heat energy is lost. The higher the G value the more free solar heat gain is obtained from the sun. Some degree of gain will be achieved in both summer and winter; however in the summer months this can lead to the property 'overheating' requiring increased ventilation by opening of windows. The lower the air infiltration rate the less heat is lost through leaky windows. When testing air infiltration rates any trickle vents fitted in windows are excluded. The air infiltration rate is the air loss through the window seals.
Now consider your home and how you heat it!
If your home gets too hot in summer, and if you need to use air conditioning units, then, a lower G value may be
better. Ensure you have adequate opening lights and trickle ventilation. If your home is particularly shaded, say
from trees, other building etc., then a high G rating will have little real world benefit. If your home needs excessive heating in winter then look for the best u-value, higher G value and lowest air infiltration rates. The best u-value is likely to have the greatest impact on saving fuel.
What exactly are U and G values?
U values are the basic measure unit for establishing the heat loss of a construction segment. They indicate the
amount of heat that passes through one m2 of a construction segment in a time unit during a temperature
differential between indoor and outdoor air of 1 degree Kelvin (W/m2K). The lower this value is the better the
thermal insulation of the construction material. A low U value is the most important characteristic of insulating glass. Low heat loss from better thermal insulation leads to higher surface temperature of the interior windowpane and thus higher comfort in the room. This enables the use of less heating equipment and a such offers both economic and ecological advantages.
G values are the coefficient of the permeability of total solar radiation energy stated as a percentage. Composed
of the direct transmission of energy and the secondary dispensation of heat of the glazed surface toward the
interior, which occurs on the basis of absorbed solar rays. The sun can heat a room only heat transmission
through the glass and thus also contribute to heating without any additional expenses. What is welcomed in winter can be unpleasant in summer, because gaining energy through solar radiation means heat. The correct course is high thermal insulation, which means a low U value combined with a G value that is not too high. The industry standard has become insulating glass units with a plated layer. Formed by
at least two panes of glass, one of which is coated with an extremely thin layer of high-grade metal, which radically reduces thermal losses.
What are Windows Energy labels?
Window Energy Labels work in the same way as household appliance energy labels for fridges, freezers and washing machines. The Window Energy Rating labelling scheme was launched in the UK by the British Fenestration Rating Council (BFRC) and is now joined by the Thermal Rating Register , both government supported independent bodies. By using information contained on the label, buyers can reliably compare
energy efficiencies of one product against another.
What Are The Benefits of 'A' Rated Windows?
'A' Rated Windows have zero heat loss, and are a huge improvement over most double glazed windows installed in the last 30 years, almost all of which will loose nearly twice as much heat as an 'A' rated window. With energy prices rising and rising, the benefit of installing an 'A' rated window will help not only the environment, but will save thousands of pounds on household heating bills. Over 20 years a saving of up to £11,420 and a saving of
22.87 tonnes of CO2 emissions & 6.24 tonnes of carbon emissions can be achieved when 'A' rated windows are installed*
*Figures have been taken from the Carbon Calculator developed by the GGF and the calculations have been verified by the Energy Savings Trust - homeowners can access the Carbon Calculator by going to www.ggf.co.uk Figures quoted are for replacing single glazing in a detached house with 23sq mt of glass and heated by gas with a fuel inflation rate of 9%