Windows are interesting and complex elements of a home. They offer views that connect indoor and outdoor living spaces.
Windows let in fresh air and sunlight. They can, however, be sources of significant heat loss during the winter and unwanted heat gain during the summer.
Energy efficient windows provide comfort, reduce energy costs, and help create a healthier, cleaner, and brighter environment.
Windows are capable of severely impacting the cooling and heating load of a building. Government sources say windows can be responsible for as much as 40 percent of the home’s loss in heating energy and 87 percent of the heat gained. Improving the thermal performance of windows reduces greenhouse gas emissions and energy costs.
Incorporating Passive Solar Principles
Glazing has a complex impact on a building’s thermal performance. There is an interaction of climatic conditions, building design, building materials, the location and size of shading and windows, and the glazing system’s thermal properties.
The wind, the sunshine, humidity, and temperature at a location are climatic conditions that have an impact on thermal performance. The building’s layout, form, and orientation are building design elements that affect the performance. Insulation and mass of building materials play a role in thermal efficiency.
The most cost-effective means of achieving high thermal performance is the incorporation of passive solar principles when designing a building. The design should include size and location of shading and windows that exclude temperature in hot weather and let the sunshine in when temperatures are cold.
Using thermal mass stores the heat of the sun and provides night-time warmth when conditions are cold. Window locations should allow natural cross-ventilation cooling. Openings should be sealed to minimise draughts.
Some sites present more of a challenge in implementing passive solar design than others. Neighbouring buildings may block the winter sun. Views may be to the west or south, leading to windows with poor orientation. In such cases, double glazed windows improve the thermal performance and compensate for detrimental building design.
Careful choice of double glazed windows improves thermal comfort near windows, especially those that are large. The sense of comfort is determined by more than air temperature. Surrounding surface temperatures have a great impact. The objective is to achieve an inside surface glass temperature that is as close to the desired temperature of the room as possible. The glass should not be hot in the summer or cold in the winter.
The orientation of double glazed windows is critical. Orientation positions a building about
variations of the sun’s path and prevailing winds that have a seasonal impact. Good orientation increases the home’s energy efficiency, making it cheaper to run and a more comfortable place to live. Optimizing double glazed window orientation is more likely to yield better results than a one-type-fits-all approach.
Thermal Efficiency Values
It is a common practice to use windows having the same solar heat gain coefficients and U-value on all elevations. The rate of non-solar heat conducted by a window system is expressed in terms of its U-value. The window assembly; that includes spacers, seals, glass, and frames; affects the U-value of windows. The lower the U-value is, the better the window’s insulating value and heat flow resistance. Double glazed windows cut the U-value approximately in half. The lower U-value also means the heat loss is also cut in half.
A window also has a solar heat gain coefficient that is the measure of readily direct heat flow through the window system. The lower the SHGC is, the less solar heat is transmitted. The best performance levels in both SHGC and U-values are achieved with double glazed windows.
Nathan Bishop is the owner Go Green Glazing. They are an Australian business based in Geelong that specialises in the retrofit of double glazed windows. His team has over 100 years of combined experience in window repair, replacement & retrofitting.