If you are thinking of buying new windows you can't have failed to be offered energy rated windows either "A", "B" or "C" especially as the national window companies see this as an opportunity to see more at a higher price. Many national window companies now offer an "A" rated window as standard, but is it really worth it?
The BFRC British Fenestration ratings council test each window and glass configuration for three things: -
1. U-Value
2. Solar Gain
3. Air leakage
The higher the number the better rating and banding. This is usually increased in 3 way
1. Low Iron Glass - this increase the Solar gain but not the u-value
2. Warm edge spacer bar - stops thermal transfer and increases u-value
3. Insulation in the outer frame
Be VERY careful here as a lot of companies supply windows that are equivalent to an "A", "B" or "C" but have not been tested with the glass they are supplying with the frames. Other have achieved an "A" rating but only with slim outerframe which is useless in 9 out of 10 properties as the bead is hidden behind the plaster line.
Are they worth it?
For the cost of it, maybe, that's up to you. If you visit Pilkington"s website there is a calculator showing the savings from single glazing to energy rated in a semi detected property. Exaggerated maybe but the saving from "C" to a "A" is only 700 over 20 years, only 35 a year! It also onlt saves 1.94 Tones of CO2 which is less than a return flight from Heathrow to New York once every 20 years.
So the answer is if you changing you windows for other reasons and you can negotiate a good price it may be worth it, but changing to save money or reduce your carbon footprint probably isn't.