Window
Window | Glazing
photo: Scartop Joinery
©yourhome.gov.au
Windows are complex and interesting elements in the fabric of a home. They let in light and fresh air and offer views that connect interior living spaces with the outdoors. However, windows can be a major source of unwanted heat gain in summer and significant heat loss in winter.
Energy efficient windows make your home more comfortable, dramatically reduce your energy costs and help to create a brighter, cleaner and healthier environment.
Windows can severely impact on the heating and cooling loads of a building. Up to 40% of a home’s heating energy can be lost and up to 87% of its heat gained through windows. Improving windows’ thermal performance reduces energy costs and greenhouse gas emissions.
Glazing and thermal performance
The impact of glazing on the thermal performance of a building is complex. Consider:
- climatic conditions in your location — temperature, humidity, sunshine and wind
- building design — the orientation, form and layout of the building
- building materials — the amount of mass and insulation
- the size and location of windows and shading
- thermal properties of glazing systems.
Passive solar design
Incorporating passive solar principles at the design stage is the most cost effective way of achieving good thermal performance (see Design for climate; Orientation; Shading; Passive solar heating; Passive cooling; Insulation; Thermal mass).At the design stage, some simple principles can be followed to optimise the thermal performance of your home.
- Locate and size windows and shading to let sunshine in when the temperature is cold and exclude it when it’s hot.
- Use thermal mass to store the sun’s heat and provide night-time warmth in cold conditions.
- Locate window and door openings to allow natural cooling by cross-ventilation.
- Provide seals to openings to minimise unwanted draughts.
Thermal comfort
Careful choice of glazing greatly improves thermal comfort for people close to windows, especially large windows. Our sense of comfort is not just determined by air temperature: the temperature of surrounding surfaces has a great impact. The objective should be to achieve an inside glass surface temperature as close as possible to the desired room air temperature. This means glass that is neither cold in winter nor hot in summer.
Choosing the right window
Windows can add to the energy performance of your home through two distinct heat transfer mechanisms — conduction and solar heat gain.
Conduction: U-value
U-value (expressed as Uw in windows) measures how readily a window system conducts heat. It is a measure of the rate of non-solar heat loss or gain through it. The rate of heat is indicated in the terms of the U-value of a window assembly which includes the effect of the frame, glass, seals and any spacers. The lower the U-value, the greater a window’s resistance to heat flow and the better its insulating value.
Solar heat gain coefficient
The SHGC for windows (expressed as SHGCw) measures how readily heat from direct sunlight flows through a window system. The SHGC is the fraction of incident solar radiation admitted through a window, directly transmitted as well as absorbed and subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a window’s SHGC, the less solar heat it transmits.
Angle of incidence
The angle that solar radiation strikes glass has a major impact on the amount of heat transmitted. When the sun is perpendicular to the glass it has an angle of incidence of 0°. For standard clear glass 85% of solar heat is transmitted. As the angle increases, more solar radiation is reflected, and less is transmitted. It falls sharply once the angle exceeds 55°. Also, as the angle increases, the effective area of exposure to solar radiation reduces.
