Low-emittance - or Low-E - windows are an increasingly common way to control temperature within the building envelope. Traditionally, windows were accepted as being “energy losers” - they lacked the insulation found in the rest of the building frame, so they were prime locations for heat to escape in winter or for cool air to escape in summer (either being conducted through the glass or slipping through cracks in the window frame).

But in the last 20 years low-E window coating has changed that. The coating is a thin film that can be applied to a normal window - at first the result was a green- or blue-tinted window, but now low-e windows can look just like traditional ones. Originally the coating was applied to the interior side of windows in cold-climate homes, because its properties prevented the conduction of heat. At the time, the technology could not be put to use in warm climates, to keep heat on the outside of the window, because the coating would have blocked transmission not only of the sun’s thermal radiation but also of its light.

Modern low-E windows, however, can be used to save energy in all climates, because the coating can now distinguish between light rays and thermal rays, permitting the sunlight to enter the building envelope while bouncing the heat back. About half of all new homes are built with low-emittance windows. When you’re in the market for a new home, be sure to look for a house with low-E windows - whether you live in a cool or warm climate, they will reduce heat transfer and energy bills, making a traditional energy loser into an environmental triumph.

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