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There are a number of coatings that can be applied depending on the end-product. One common coating is low emissivity (Low E) coating, which could go on to improve the energy performance and comfort of your space. Low-E is a clear glass which has a special coating on one surface of the glass. Low-E refers to low emissivity and this describes the capacity of a surface to radiate heat. Emissivity is measured across a scale from 0 to 1 with 1 representing the highest emissivity.

Low-E coatings have been developed to minimize the amount of ultraviolet and infrared light that can pass through glass without compromising the amount of visible light that is transmitted.

When heat or light energy is absorbed by glass, it is either shifted away by moving air or re-radiated by the glass surface. The ability of a material to radiate energy is known as emissivity. All materials, including windows, radiate heat in the form of long-wave, infrared energy depending on the emissivity and temperature of their surfaces. Radiant energy is one of the ways heat transfer occurs with windows. Reducing the emissivity of one or more of the window glass surfaces improves a window's insulating properties. 

This is where low emissivity (or Low-E glass) coatings become a popular choice. Low-E glass has a microscopically thin, transparent coating (it is thinner than a human hair) that reflects long-wave infrared energy (or heat). Some Low-E windows also reflect significant amounts of short-wave solar infrared energy. When the interior heat energy tries to escape to the colder outside space during the cooler months, the Low-E coating reflects the heat back to the inside, reducing the radiant heat loss through the glass. The reverse happens during the summer. To use a simple analogy, Low-E glass works the same way as a thermos. A thermos has a silver lining, which reflects the temperature of the drink it contains. The temperature is maintained because of the constant reflection that occurs, as well as the insulating benefits that the air space provides between the inner and outer shells of the thermos, similar to an insulating glass unit. Since Low-E glass is comprised of extremely thin layers of silver or other low emissivity materials, the same theory applies. The silver Low-E coating reflects the interior temperatures back inside, keeping the room warm or cold.

Thank you to the Vitro Architectural Glass Education Centre in the USA and Viridian Glass for some of the info used to explain Low-E glass. 

SOURCE: Viridian Glass

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