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Compiled by FaciliesNet Staff
Solar heat gain through windows is responsible for roughly one third of a building's cooling load, according to the Department of Energy. That makes applying solar window film a logical step in reducing solar heat gain and improving the energy efficiency of a building.
Solar window film is applied to the inside of a window where it reflects and absorbs heat from sunlight. In addition to reducing the need for cooling, window film helps create an even, comfortable temperature throughout a building.
A building’s directional orientation relative to the sun can create “hot spots,” or areas that receive significantly more sunlight than others. Depending on the sophistication of a facility’s HVAC system, this can create challenges for maintaining occupant comfort.
Solar control window film reduces heat gain by blocking solar radiation. Solar radiation, or solar energy, is made up of three components: ultraviolet radiation, visible light and near-infrared radiation. Near-infrared radiation makes up 53 percent of the solar spectrum, visible light 44 percent, and ultraviolet 3 percent. When rays from the sun hit a window, some of the energy is absorbed and some is reflected by the window, but most is transmitted through the glass.
According to the International Window Film Association (IWFA), a pane of clear glass reflects about 6 percent of solar radiation, absorbs 5 percent and transmits the remaining. When this energy enters a space and is combined with the ambient heat already present in a room — from people, computers and reradiated heat absorbed by furniture and carpeting — the excess heat can make an uncomfortable difference, particularly for people near windows.
Installing solar control window film increases the amount of solar energy both reflected and absorbed by the window. Most window films have a thin metallic coating, made up of aluminum, stainless steel, silver or a combination, that reflects and absorbs solar radiation. Of the radiation absorbed, most of it is reradiated outward, though some is radiated into the building.
The most important performance measurement of the heat rejection ability of solar control window film is the solar heat gain coefficient (SHGC), says Darrell Smith, executive director of the International Window Film Association. SHGC measures the total amount of solar energy transmitted into the room. This includes both solar energy directly transmitted through the glass and solar energy absorbed and radiated into the building by the film. The heat rejection ability of window film can be as high as 80 percent, says Smith. The amount of energy reflected and absorbed varies depending on the types and quantities of metals used in the film.
The near-infrared and visible light rejection capabilities of window films vary, and the combination that is best will be determined by the goals for the application, and what results are desired.
A reduction in solar heat gain can translate directly into fewer kwh used for cooling. What’s more, most utilities have a separate demand charge based on the highest monthly rate of electricity use for a year, or the peak demand. This charge is in addition to the regular cost of electricity. By reducing the amount of cooling needed during peak periods, demand charges can also be reduced.
What’s more, window film can improve occupant comfort as they save energy. Areas near windows are sometimes too warm because of the amount of heat coming in from sunlight. By reducing the solar heat gain, window films can make those perimeter areas more pleasant for people using the spaces.
Some utilities offer rebates and other incentives for installation of window film because of its ability to help save energy. A facility executives considering a window film installation should check with the local utility to see if incentives are available or planned.