What Is Low-Emissivity Window Coating?

Low-Emissivity Coating

• Low-E coating is microscopically thin and composed of various chemicals and metallic oxide materials. There are three types of low-E coatings: high-solar gain, moderate-solar gain and low-solar gain. Solar gain is the amount of sun light that transmits through glass into the room or space behind the glass. In general, the darker the coating and glazing, the less a room or space will gain solar light. You can feel solar gain as heat. This is why low-E coatings of various levels (high, moderate and low) are selected to control heat loss and heat gain.

High-Solar Gain

• High-solar gain, low-E windows are designed to reduce heat loss but admit solar gain. This type of low-E coating is best suited for cold climates where keeping heat indoors is the goal. For example, those who reside in states including Minnesota, Wisconsin and the Northeastern region of the United States can benefit from installing high-solar gain, low-E windows in homes and offices. This type of low-E glass is also used for solar heating systems.

  High-solar-gain, low-E glass is made with a pyrolytic coating and referred to as a “hard coat.” The coating is chemically bonded to the glass at the molecular level while the glass materials are still in a semi-molten state. A high-solar-gain coated, double-pane window with a U-factor of 0.30 will allow 71 percent of solar light from outdoors to transmit to the indoors and provide visibility of 75 percent of the light from outdoors.

Moderate-Solar Gain

• Moderate solar-gain, low-E windows are used in areas that have both heating and cooling issues. This includes residential and commercial applications for those in states such as Kansas, Missouri and areas where winters are cold and summers are hot. Moderate solar gain, Low-E windows are called “spectrally selective” low-E glass. They reduce heat loss and allow a reduced amount of solar gain to transmit through the window. A double-pane, low-E window with a moderate-solar-gain coating might have a U-factor of 0.25, and allow 39 percent of solar light to transmit through the window -- you could see 70 percent of the light from outdoors.

Low-Solar Gain

• Low-solar gain, low-E glass coatings reduce solar heat gain and provide the highest visibility of outdoor light. Like moderate solar-gain coatings, low-solar gain, low-E coatings are spectrally selective. The low-solar gain coating is made from a process called “sputtering.” In this manufacturing process, chemicals are deposited on the exterior layer of the glass to produce properties that reduce solar heat transmission. Low-solar gain windows reduce solar heat gain to 64 percent and allow only 27 percent of visible light to enter. Low-solar gain coatings are commonly referred to as “soft coats” and are best for cooling-driven climates, such as areas of the South and the Southwestern region of the U.S.