Low-emissivity (low-e) glass has a virtually invisible microscopically-thin coating made of a semiconductor or metallic material applied to one of the inside surfaces of a double-paned window, or on a transparent sheet suspended between the two panes. Low-e coatings are now standard options from all major window manufacturers.

The Low-e coating is usually placed on the surface of one of the panes adjacent to the trapped air space. The coating is usually applied to different surfaces for different climates. It absorbs and reflects radiant heat and ultraviolet rays.
Low-e works by acting like a mirror, reflecting 40% to 70% of long-wavelength (heat) radiation back into the heated indoors. Yet, unlike a mirror, it allows the full amount of short-wavelength solar radiation (sunlight) to pass through.
In reflecting untraviolet rays, Low-e windows reduce fading of interior furniture and carpets and helps reduce the deterioration of the window frame.
Low-E glass is at its most efficient in cold winter months where highest savings in energy costs will be noticed. The U-value for a double-paned window with Low-e glass starts at about 0.4. If a Low-e film is suspended between the glass panes, then U-values approach 0.333 and even more of the harmful ultraviolet rays are blocked.
During the warmer season, Low-e glass reflects about 25% more heat back outside than a single-paned window and 11% more than a standard double-paned window. A newer type of Low-e glass, referred to as "southern" Low-e glass, is designed to be most effective during the summer, by reflecting even more of the sun's heat away from the inside of the home.