Energy-efficient windows help reduce your heating costs, increase comfort and be environmentally friendly because of their lower carbon footprint. They can be made using any frame material or combination of materials. A useful tool for building alternative energy efficiency in windows is the UK Door and Window Rating Board [BFRC], which evaluates window energy efficiency. This is a national system that complies with current building codes for new or replacement window installations. However, the plan is still voluntary.
The energy rating scheme uses traffic light energy labels, from A to G, similar to the energy labels on most home appliances. Grade A graduated windows are the most energy efficient. Each window rated by the BFRC has a unique label that shows the overall rating [AG] and energy index, showing how much energy will be saved or lost after the window is installed. It depends on many factors such as the building, the local climate and the indoor temperature. A positive value in the index indicates that the window enters the room through the glass with more heat than it loses through the window. Such a window is a free energy supplier. Class A windows will have a positive energy index. The value is in kilowatt hours per square meter per year.
The tag also displays the U and L values of the window. These are indicators of the ability of windows to reduce heat loss during indirect radiation exposure during winter. A low U value means less heat loss and translates into lower heating costs. The L value indicates the effective heat loss due to air permeation. Another value shown is the solar heat gain. This value is closely related to the U value and measures the ability of the window to reduce heat gain during direct radiation exposure during the summer. A lower value means less direct heat enters the room and continues to cool.
The type of glass used directly affects the U value and the solar gain factor. The most energy efficient glass for double glazing is low emissivity [Low-E] glass. This typically has an invisible metal oxide coating on one of the two sheets of glass from which the window is made, usually on an inner panel. This causes the glass to reflect, allowing sunlight and heat to enter and reducing the amount that can come out again, thus warming the family in the winter. In the summer, it reflects the heat back and makes your home even cooler.
Very energy-efficient windows may use argon, helium or neon gases in the gap between the glass plates. More efficient windows will also use the pane spacers around the inside edge to keep the two glass panes separate, with little or no metal.
Choosing energy-efficient windows has become easier because you can compare window energy levels. The window energy rating scheme examines all components of the window [including the frame and any sashes] and rates the energy-saving characteristics of the entire window. Windows rated B or higher is approved by the Energy Conservation Trust. Energy Labels allow you to compare and select windows based on specific energy performance, rather than relying solely on manufacturers and statements, allowing you to make informed purchasing decisions.