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 reradiated by the glass surface. The ability of a material to radiate energy is known as emissivity. In general, highly reflective materials have a low emissivity and dull darker colored materials have a high 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 important 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 come into play. Low-e glass has a microscopically thin, transparent coating – it is much thinner than a human hair – that reflects long-wave infrared energy (or heat). Some low-e’s also reflect significant amounts of short-wave solar infrared energy. When the interior heat energy tries to escape to the colder outside during the winter, 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 time.. To use a simple analogy, low-e glass works the same way a thermos does. A thermos has a silver lining, which reflects the temperature of the drink it contains back in. 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.
There are actually two different types of low-e coatings: passive low-e coatings and solar control low-e coatings. Most passive low-e coatings, are manufactured using the pyrolytic process – the coating is applied to the glass ribbon while it is being produced on the float line, the coating then “fuses” to the hot glass surface, creating a strong bond, or “hard-coat” that is very durable during fabrication. Finally, the glass is cut into stock sheets of various sizes for shipment to fabricators. Passive low-e coatings are good for very cold climates because they allow some of the sun’s short-wave infrared energy to pass through and help heat the building during the winter, but still reflect the interior long-wave heat energy back inside.