Energy-efficient windows help to reduce your heating bill, increase comfort and are environmentally friendly as they have a lower carbon footprint. They can be made using any frame material or combination of materials. A useful tool to establish whether replacement windows are energy-efficient is the British Fenestration Rating Council's (BFRC) scheme for rating the energy-efficiency of windows. This is a national system and conforms with the current building regulations for new or replacement window installations. The scheme is, however, still voluntary at present.
The energy rating scheme uses traffic light energy labels for windows on a scale from A to G that are similar to the energy labels you can find on most household appliances. A-graded windows are the most energy-efficient. Each window rated by the BFRC has a unique label displaying the overall rating (AG) and the energy index which shows how much energy the window will save or lose once it has been installed. This depends on a number of factors such as the building, local climate and indoor temperature. A positive value in the index indicates that the window lets more heat in through the glass into the room than is being lost through the window. Such a window is a free energy supplier. A-rated windows will have a positive energy index. Values are given in kilowatt hours per square meter per year.
The label also shows the window's U and L values. These are measures of a window's ability to reduce heat loss during indirect radiation exposure during the winter. Low U values mean less heat loss and translate into lower heating costs. The L value indicates the effective heat loss due to air penetration. Another value shown is the solar heat gain. This value is closely linked to the U value and measures the window's ability to reduce heat gain during direct radiation exposure in the summer. A lower value means less direct heat enters the room and it stays cooler.
The type of glass used has a direct impact on the U value and the solar gain factor. The most energy-efficient glass for double glazing is low emissivity (Low-E) glass. This often has an invisible coating of metal oxide on one of the two sheets of glass that make the window, normally on one of the internal panes. This makes the glass reflective, letting sunlight and heat in and cutting the amount that can get out again, thus making the home warmer in the winter. In the summer it reflect the heat back and makes your home cooler.
Very energy-efficient windows might use gases like argon, xenon or krypton in the gap between the sheets of glass. The more efficient windows will also use pane spacers around the inside edges to keep the two panes of glass apart that contain little or no metal.
Selecting energy-efficient windows has become easier as you can compare window energy ratings. The window energy rating scheme checks all components of the window (including the frame and any casement) and rates the whole window on its energy-saving properties. Windows rated B or better are endorsed by the Energy Saving Trust. Energy labels allow you to compare and select windows based on their specific energy performance rather than just relying manufacturers' claims, thus enabling you to make informed purchasing decisions.