[Greenbuilding] South-facing windows are net energy gains

[Doug Kalmer]

Windows That Perform Better Than Walls
If you choose the right glazing, your windows can gather more energy than they lose
The common perception that windows are "energy holes" is a bad rap. Since today's high-solar-gain triple-glazed windows gather more heat than they lose, good windows perform better than an insulated wall. After all, a wall can only lose energy, while windows can gain energy during the day to balance energy lost at night.
The Canadian window-rating system
In Canada, windows are rated according to the ER (Energy Rating) method. For those who live in cold climates, the Canadian ER system is arguably an easier-to-understand method of rating windows than any system used in the U.S., where NFRC window labels cause a fair amount of head-scratching. 

As explained on a Natural Resources Canada Web site, "A window's ER rating is a measure of its overall performance, based on three factors: 1) solar heat gains; 2) heat loss through frames, spacer and glass; and 3) air leakage heat loss. A number is established in watts per square meter, which is either positive or negative, depending on heat gain or loss during the heating season."

While windows with a negative ER are "energy holes," windows with a positive ER act like heaters. Poorly designed windows can have ER ratings as low as -25, while the best-performing triple-glazed windows have an ER of about +1 (for operable windows) or +8 (for fixed windows). 

South-facing windows produce the most energy
Consider a 1-square-meter fixed window equipped with the best available triple glazing installed in southern Canada. Over the course of a heating season, this window will, on average, perform like an 8-watt heater. This level of performance is an average based on all four orientations; if the window faces south, it will contribute much more than 8 watts.
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