Windows and doors let sunshine in and let people and things in and out of a home or building; however, they both contribute to energy loss, through a technical process called fenestration. According to the California Energy Commission, close to 30 percent of your energy dollars leave through your doors and escape through your windows. However, by using double-pane and low-E coated windows and doors you can reduce heat loss in the winter and heat gain in the summer.
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A double-pane glass is a term to describe the process of adhering two pieces of glass together to create a window or door. There is a pocket of airspace in between the two pieces of glass, which is dried so that the air space is airtight to eliminate the possibility of condensation and to maximize the insulation properties. In cases when the glass will be low-E coated, the pocket is typically filled with argon gas to prevent condensation. The panes are inserted inside of a window frame, which is typically made of metal. The glass facing the outdoors can be tinted so that those outside cannot see inside, while the glass facing the interior is left clear so those inside can see outdoors. The second option is to keep both panes of glass clear, and with or without a low-E coating on the exterior glass pane.
Low-E is a microscopically thin and virtually invisible coating that is applied to the side of glass that faces the outdoors. A low-E coating improves the efficiency of the glass at reflecting a desired level of heat away from the building and allowing a desired level of solar heat to pass through the glass and enter the building. The technical term used to describe this process is "low-emittance." The coating is made from metal or metallic oxide materials, which are sprayed onto glass.
Double-pane clear and low-E coated glass are rated for energy efficiency based on the U-factor, solar heat gain factor, and visible light transmittance. The U-factor measures the thermal performance of the glass in terms of how well it transfers heat into or out of the building. All of the window components, including the frame, sash and glass, are taken into account in calculating the U-factor. In general, the lower the U-factor, the higher the energy efficiency of the glass. The solar heat gain factor, or SHG, is a measurement to calculate how the glass performs at cooling heat gains when the glass absorbs solar energy. The visible light transmittance is a ratio measurement used to express the amount of light that you can see through the window. Double-pane windows are low-e coated to produce a high, medium or low SHG factor. For example, double-pane high-solar gain low-E glass is used in heating-dominated climates, such as areas in the Northeast and Midwest, medium solar gain low-E glass is used in the Midwest where there's a need for both heating and cooling, and low-solar-gain low-E glass is used in western and southwestern regions of the U.S.
The term double-pane refers to a window that has two pieces of glass, while low-e is a coating technique. Double-pane windows can be clear, tinted or have a low-e coating. You can have glass with or without a low-e coating. However, low-e coating is not a standalone material. It requires a sheet of glass as a base material for its application.