Today, builders and renovators aim for energy efficiency, particularly in areas where people must heat their homes for significant periods of the year. Because a lot of heat can be lost through windows, keeping the heat in during the colder months means using special technology. The current standard energy efficient window is made with Low-E glass (also written LoE, low-e).
What is low-e glass?
Low-e stands for low emissivity: low-e glass windows emit a relatively low amount of heat to outside air during the heating season. Modern low-e windows also block some of the heat from entering the home during hot summer weather, keeping the inside of the home cool and saving on air conditioning costs.
Low-e glass is coated with an extremely thin layer of metallic oxide. This coating does not obstruct vision or prevent visible light from passing through the window, but it does affect passage of both ultraviolet (UV) light and infrared (IR) light. Various metallic oxides with different properties are used. They can absorb, reflect, or transmit IR radiant heat, depending on their composition.
In windows with several panes of glass, argon gas is often used to fill the space between panes. Argon gas, in combination with low-e glass, helps to prevent heat from escaping through windows.
U-value and solar heat gain coefficient
Various ratings are used to describe the energy efficiency of windows. Two useful measurements are the U-value (or U-factor) and the solar heat gain coefficient (SHGC):
- The U-value describes the amount of heat that is lost through the glass – the lower the U-value the less heat passing through.
- The SHGC tells you how much of the solar heat striking the exterior surface of the window is allowed to pass through into the interior. The lower the SHGC rating, the less heat passing through.
In a cold climate, where it’s desirable to keep heat in on the majority of days, a low-e window with a low U-value and a relatively high SHGC is desirable. In a warm climate, the reverse is true. A study done by the National Research Council Canada (NRC) showed that, for regions where homes are heated for a majority of the year, greater energy efficiency is realized with a high SHGC, even when a small sacrifice is made in U-value.
The NRC study found that for all Canadian "energy star zones," low-e windows with high SHGC values are the best choice.
Sources
- Barry, C.J. and A.H. Elmahdy. “Selection of optimum low-e coated glass type for residential glazing in heating dominated climates.” National Research Council Canada: NRCC-50253
- Building Energy Codes Resource Center. “What is a Window U-factor?” and “What is a Window SHGC?” resourcecenter.pnl.gov
- Dominion Window and Door Ltd. “Understanding Low-e Coatings.” dominionwindow.com
Rosemary Drisdelle - Rosemary Drisdelle has been published many times as a nonfiction writer and several times as a poet. Her first book, Parasites: Tales of ...
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