The exact energy savings on fitting high performance 'A'-'B' rated windows depends on various factors, such as the specification of the actual window frame, the glazing itself and the type of window being replaced. However, high performance windows can save thousands of £'s so the option is definitely worth exploring!
The Glass and Glazing Federation website hosts an independent Energy Savings Calculator which allows you to input the details of your current windows to calculate the yearly savings on Energy, CO2 and Carbon you can gain by upgrading.
Once you've made the decision to look at high performance windows, the next stage is to look at the options on the market. Firstly, look for the Energy Rating label as per the image on this page. It is a similar design to the ones that you may be familiar with currently displayed on appliances such as washing machines and dishwashers showing various colour bands and indicative 'A' - 'G' ratings.
There are various Government Approved Competent Persons Bodies operating within the Window Industry who can endorse these labels. The main ones are the BFRC (British Fenestration Rating Council), CERTASS and the BSI.
The Energy Saving Recommended logo as shown on this page is the mark of the Energy Savings Trust and is also an endorsement that your window is one of the most energy efficient windows (B and above).
Pre-October 2010, the minimum requirement for replacement windows as dictated by UK Building Regulations was an 'E' rated window. This then changed to a minimum of a 'C' rated window (please note that some window installers may offer an equivalent whole window U-value which is also accepted for Building Regulations). Proposals suggest that this will be raised to a minimum of a 'B' rating in 2013.
The next aspect of choosing your new windows is to make sure you buy quality to last. In terms of double glazing, a glazing unit is expected to last up to ten years, but if it fails the cost of replacing a sealed unit that has misted up will far outweigh the potential energy savings and subsequent environmental impact.An 'A' rated window means that there is theoretically no energy lost through the window. This saving is great, but insignificant if a glazed unit fails due to poor quality. A double glazed unit is expected to last up to ten years, but if it fails the actual cost of replacing the sealed units is likely to be more than the theoretical energy lost over a decade. So whatever style of window you choose, insist on high performance Thermobar glazed units to ensure you get the longest possible life out of your new environmentally-friendly windows.