Windows are a critical component in sustainable building design. Most energy flows in and out of a building through its windows, but with intelligent design and technological innovation they can be used to provide heating, cooling and lighting for a better indoor environment.
To optimise the energy performance of windows the first consideration is passive design. Natural light and heat flow through a window can be controlled to some extent through appropriate size and solar orientation. Larger windows have more potential to lose or gain heat than smaller windows, and south-facing windows let through more heat and light than north-facing windows. Awnings and overhangs can be incorporated with window design to allow sunlight in winter and shade in the summer.
Window Energy Ratings
There are many different types of window available, which can be confusing for the consumer. Fortunately the British Fenestration Ratings Council (BFRC) has calculated an energy performance rating for windows using a label to classify windows from A to G (where A is the most energy efficient window). These ratings are not absolute since they do not take into account the size and orientation of a window, but they are a good indication of overall performance. The values used in calculation are:
- U-Value – a measure of heat loss, also known as thermal transmittance. A low U-factor is best, and UK building regulations currently specify that it should be less than 3.3 W/m2K (where watt is a unit of energy, m2 is the area of the window, and Kelvin is the difference in temperature from outside to inside)
- Solar Heat Gain Co-efficient (SHGC) – also known as solar factor, or g-value, measures how much a window will absorb heat from sunlight. It is a number between 0 and 1, where a low SHGC means less heat gain.
- Air Leakage (L50) – measures how much air leaks in and out of a window when it is closed.
The BFRC rates the energy performance of a whole window, by considering both the frame material and glass characteristics.
Frames can significantly affect a windows performance, depending on the type of frame material. Wood frames are energy efficient, and can be sourced from a certified sustainable supplier, but can also be expensive and need to be painted regularly to protect them from water damage. Fibreglass frames are low maintenance and energy-efficient but can also be expensive. Aluminium frames are inexpensive but conduct heat easily and are therefore less energy efficient than other options.
One of the most popular materials in the UK for framing is PVC, also called uPVC, and PVCu. These are made from vinyl and are inexpensive, energy efficient and low maintenance. They do not need to be painted and are highly resistant to moisture. Some vinyl frames have been insulated to give improved thermal performance.
Energy efficient windows have two or more panes of glass separated by an air gap to slow heat transfer and provide noise insulation. These are known as double paned or insulated windows. Traditionally air or dry nitrogen was used between the panes, but recent models use low conductance gas fills of argon or krypton that gives even better thermal performance and significantly lowers the U-value.
Another new development in window technology is low-emittance coating also known as low-e. These are microscopically thin layers of metal or metallic oxide that are coated onto the window glazing. Different types of coating have different properties, but all are designed to lower heat flow through the window – some reflect heat back into the building and others reflect heat back outside.
Spacers are used in between the layers of glass in a window, to hold them together at the required distance. Conventional spacers were made from aluminium, but because of their high conductivity they provided poor thermal insulation and created condensation problems around the window edges. New spacer products, known as warm spacer technology, or warm edge spacers, have been developed from materials that have a low conductivity rate and high thermal efficiency, and include fibreglass, vinyl, silicone foam and reinforced thermoplastic. Some warm edge spacers incorporate a thermal break in their structural design.
Saving Money, Saving the Planet
Window technology has undergone a revolution in the last twenty years. Using a combination of gas filled double-glazing, glass coatings, thermally improved edge spaces and PVC framing it is now possible to adjust solar gain, reduce condensation, prevent air leakage and improve the comfort of an indoor environment while offering the homeowner huge savings in energy and money. More importantly, energy efficient windows reduce carbon emissions and help to address global climate change. Used in combination with other green building methods and materials, they are a good choice for a brighter, sustainable future.