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Heat loss through glass windows is a major problem for home designers. Unlike heat loss through the roofing which can be prevented using thick and heavy insulation glass presents a problem as it needs to be transparent and to a certain thickness to enable it to fit into standard window frames.
The energy efficiency of any insulation can be judged by its U-Value rating. This is a measure of how well the material prevents heat from escaping (thermal conductivity, with a high value being better i.e. more insulated).
To simplify things the British Fenestration Rating Council (BFRC) introduced a rating system of A to G, (where A is the most energy efficient window, based on U-rating, Air leakage and g-value (a measure of how well a window blocks sunlight).
These ratings are not final as they don't take into account the size and orientation to the sun of a window, but they are a useful indication of overall performance.
Values used in calculation are:
The BFRC rates the energy performance of the whole window system, by including the frame material and glass characteristics.
This gives the familiar A to G rating system and allows consumers to see at a glance how efficient the window is compared to others.
The government of England and Wales introduced new measures in 2002 to try to reduce the wasted energy from the nation's homes.
This meant that all new windows installed to existing or new buildings had to meet energy efficiency standards of a D rating or higher.
Various methods have been developed over the years to combat the heat loss through glass. The most common and widely used of these is double glazing.
This is not the only option available to home builders as different types of window are available made from varying materials, different coatings and production techniques.
Double glazing has now become a standard installation on new built homes. Instead of using a single sheet of glass, two panes of glass are used. The insulating properties are further enhanced by filling the gap between the two panes with an inert glass such as Argon. Double glazing can reduce the amount of heat lost by 50%, which is equivalent to around 80-100 GBP.
This reduction of heating used can save a household around 740kg of carbon emissions each year compared to single glazing. Double glazing also provides excellent sound insulation compared to single paned windows.
Triple glazing adds an extra layer to double glazing. This increases the insulation effect of the glazing by up to 40% when coupled with low-emissivity coatings. This is popular in Scandinavian countries. Triple glazing is more expensive then double glazing (around 20- 40% more) and weighs a bit more then double glazing so consideration needs to be given to the strength of the window panels. If cost is an issue then it may be worth considering triple glazing the most used/heated rooms and using double glazing on other lesser used rooms (i.e. bathrooms).
Triple glazing also increases sound insulation if the third layer is spaced at a suitable wavelength (about 10 cm) and is often favoured for homes in close proximity to noisy environments like airports.
Glass loses heat by the transfer of thermal radiation from a warm surface to a colder one. Low-emissivity or low-e coatings are a microscopic layer of metal or metal oxides placed onto the glazing which reduce the flow of radiated heat through the window as the metal layer reflects the heat back and acts as a barrier between cold and warm surfaces.
The film is completely invisible to the naked eye and while blocking thermal radiation from leaving the room it still allows solar radiation in, which in turn warms up the room.
Darkening glass or 'smart glass' refers to a type of glass that can change its properties based on the situation Smart glass can change from transparent to various levels of opacity depending on what sort of material and voltage is put through it.
This can be used to vary the amount of solar radiation allowed into a structure allowing it to be kept cooler in the summer. Smart glass technology is not as green as other options not only consuming energy in use but also more through its production. It is also created using fresh materials as opposed to greener recycled glass.
A newer offshoot of smart glass technology is the vanadium oxide coating. The coating works by chemically changing at 29 degrees Celsius into a state that blocks the sun's infrared rays preventing heat from building up and reducing the amount of cooling required in hotter months. This film can be added to existing windows saving the energy and cost of producing new windows.
With it being claimed that 25% of all house heat can be lost through poorly insulated windows it is essential not only for the environment but also for fiscal reasons to insure that all available insulation methods are considered carefully before commencing a project.
See also our DIY Building general advice on double glazing >