The Glassroom

Residential: Improving U-Factor Benefits of Fourth Surface Low-E

U-Factor measures the rate of heat gain or heat loss due to a difference in temperature between the indoor and outdoor environments of a home. The U-Factor of a window is greatly influenced by the emissivity of the glass. The emissivity of a material is the relative power of its surface to emit heat by radiation. Clear, uncoated glass will radiate equally from both surfaces under equal conditions (equal conditions being defined as the same temperature on the outside and inside of a building). If a Low-E coating is added to one surface, this surface will radiate from 75% to 96% less (depending on the type of Low-E coating), than the uncoated glass surface. If this coating is facing the airspace (the space between the two panes of glass) of a window, it will reduce the radiation across the airspace and improve the U-Factor. If the coating is facing the room side, it will reflect most of the energy radiated by room surfaces back into the room, thus further improving the U-Factor.

The main way to influence the U-Factor of a window is to utilize a Low-E coating with a lower emissivity number, increase the number of panes of glass (double-insulated glass units to triple-insulated glass products), and introduce Low-E coatings on more glass surfaces.

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