When considering glazing, a balance must be achieved between views, ventilation and heat loss and heat gain.
Windows and other glazed external doors have a major impact on the energy efficiency of a building. If not designed correctly they can allow substantial unwanted heat transfer between the interior and the outdoors. If designed correctly, they will help maintain year-round comfort levels in a home.
Basically, glazing must be positioned, sized and protected to get the most benefit from the winter sun while avoiding overheating in summer and heat loss in winter.
Heat loss and heat gain in a well insulated home occurs mostly through the windows.
In summer, each square metre of glass in direct sun can allow as much heat in as would be produced by a single bar radiator. In winter, losses from a window can be ten or more times the losses through the same area of insulated wall.
In winter, windows are used to trap the sun’s heat and warm up the spaces behind. The penetration of the sun into the house is desirable in climates with cool to cold winters. Summer sun penetration is undesirable in all climatic zones in Australia.
Windows must be well shaded in summer. The most effective is external shading in the form of an eaves overhang and pergolas with angled louvres or deciduous vines. Trees and high bushes or operable screens are an effective form of protection on west and east windows.
Savings on energy bills can quickly repay an initial extra investment in energy-efficient windows, such as low-e coating or double glazing. Getting it wrong can turn your windows into an energy liability for the life of the building.
Choosing the right glass is a major factor in determining the energy efficiency of a window.
Glass products are generally classified as being either absorbent or reflective. Solar radiation that is not reflected or absorbed is transmitted through the window.
Tinted or “toned” glass is an absorbent glass. Toned glass acts like sunglasses to reduce the solar radiation entering the home, keeping it cool.
Reflective glass has either a vacuum-deposited metal coating or a pyrolytic coating. Solar radiation and light are reflected off the glass.
Spectrally selective glazing is commonly used for westerly elevations where solar control and natural lighting are a priority. Spectrally selective glazing maximises light transmission while simultaneously reflecting unwanted solar radiation.
Low emissivity (low-e) glass has a coating that allows short wavelength energy (daylight) from the sun to pass into the house but reduces the amount of long wavelength energy (infrared heat) that can escape through the window, preventing heat loss from the interior of the house.
Single glazing offers little resistance to the passage of heat.
Double glazing offers much better insulation. It comprises two panes of glass with a sealed space between. The space is filled with air or an inert gas with better insulating properties than glass. The best thermal performance for air-filled units occurs when the space between the panes is about 12mm.
Things to consider when placing windows are
- Appropriate glass area for each wall orientation
- Using high level windows or skylights to increase solar gain and natural light
- Window opening type for good cross ventilation
- Shading devices to control heat gain and heat loss
- Thermal properties of the complete glazing system