The Energy Star Rating
In Canada, the energy performance of a window is measured through the Energy Rating (ER) system. The ER rating takes several of the factors listed above into consideration including U-Factor, SHGC and leakage. More details about this are provided below. A higher ER number on a window indicates more energy efficiency. Several accredited associations in Canada use this standard and you should always check carefully for a certification label on your product.
The ER system accounts for the following factors:
Window effectiveness in energy efficiency is measured by:
U-Factor: A factor/rating that outlines the window insulation effectiveness/rate of heat loss from the window. A window with a lower U-Factor has higher energy efficiency. The U-Factor is not limited to the glass part of the window but also includes other components, including the frame and spacers. Annual heating and cooling energy demand decreases by 7.9–16.7% when changing the U-value of the windows in a poorly insulated house.
Solar Heat Gain Coefficient (SHGC): A factor that reflects how much solar heat the window absorbs. A lower SHGC coefficient means less solar heat gain which can prove to be beneficial but also a drawback depending on your climate and your personal preferences. Windows to the East and West have the highest SHGC due to maximum exposure to the Sun and therefore, if your windows face this direction, including a shading option can be beneficial to minimize heat gain during summer. If your air conditioning load is particularly high, it is important to consider whether SHGC can be driving up your heating in the summer and use low SHGC windows. In Canadian winters, this feature can however decrease your heating costs - so it is a delicate balance. The SHGC affects your cooling load more than your heating load.
Image of Energy Star label. Image obtained from Government of Canada, click on image for reference.
Together, these two factors as well as your window orientation can determine how well your windows perform. Both of these are typically always included in the performance label on windows. Another additional feature that may or may not be included, based on manufacturer preferences, is the air leakage.
Air Leakage: Generally expressed as the amount of air flowing through a specific square area per unit time. Air leakage inevitably drives up energy costs during both heating and cooling seasons and it needs to be minimized.
1. Natural Resources Canada. (2020, September 18). Government of Canada - Ratings and Certification. Retrieved from https://www.nrcan.gc.ca/energy-efficiency/energy-efficiency-products/product-information/windows-doors-skylights/ratings-and-certification/13978.
2. U.S. Department of Energy. (2010, October). Guide to Energy - Efficient Windows. Retrieved from https://www.energy.gov/sites/prod/files/guide_to_energy_efficient_windows.pdf
3. Efficient Windows Collaboration. (2020). Measuring Performance - U Factor. Retrieved from https://www.efficientwindows.org/ufactor/.
4. Efficient Windows Collaboration. (2020). Measuring Performance - SHGC. Retrieved from https://www.efficientwindows.org/shgc/