Energy Efficient Glass
Low Emmissivity Coatings - What Are They?
Low emmissivity (Low-E) glass coatings reflect invisible long wave radiation from radiant heat sources. Clear glass has an emmissivity of around 0.84, meaning that it absorbs and emits about 84% of the long-wave radiation that strikes the glass surface. Glass with a Low-E coating on one surface will have an emmissivity from about 0.35 to as low as 0.04, thus reflecting back 65% to 96% of the long-wave radiation. Low-E coatings improve the window U-value by reflecting long-wave radiation, rather than absorbing and conducting the heat through the glass. Since a lower emmissivity results in a lower U-value, there is a real difference between a "Mid-E" emmissivity of 0.20 or higher versus a truly low emmissivity value below 0.10.
The difference between Low-E glass and Spectrally Selective Low-E glass:
In general, low emmissivity coatings are designed to reduce the heat transfer caused by long-wave radiation. As you may know, the sun's rays contain energy in various spectrums or wavelengths. Once solar radiation passes through glass it stikes the interior surfaces in the room and is absorbed and then emitted as long-wave radiation. Standard Low-E glass reflects that long-wave radiation and reduces heat loss from the room. Spectrally selective Low-E glasses (like the PPG Solarban® 60) also block long-wave radiation, but they have another important function. The multiple layers of silver in the coating allow the glass to selectively transmit and reject certain wavelengths of solar radiation. Spectrally selective Low-E coatings are designed to maximize the transmission of visible light and to reduce transmission of longer wavelength heat in the near-infrared spectrum.