| How Window Film Works
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. 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 — due to people, computers and re-radiated 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. Of the radiation absorbed, most of it is re-radiated 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.
It is important to note that the percentage of solar energy rejected and the percentage of near-infrared radiation rejected are different measurements. A measurement of near-infrared rejection only takes into account radiation in the near-infrared range, while solar energy rejection measures both near-infrared and visible light rejection. Visible light, when transmitted through glass and absorbed by furniture or carpet, also contributes to heat gain.
For example, says Smith, if two window films have an equal SHGC, and one rejects more near-infrared radiation than the other, it would also transmit more visible light. This is because more of the solar energy rejected is in the near-infrared range.
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.
“From a building management perspective, you can reduce kwh, make tenants more comfortable and reduce demand charge,” says Jim Mannix, business development manager, 3M.
Because of its ability to help save energy, window film is recognized and encouraged as an energy-efficient retrofit.
“The ability to reduce energy costs for buildings is widely accepted by many utility companies that offer significant incentives and rebates for installation of window films,” says Debusk. “Window films are used extensively by energy service companies that implement energy-efficiency improvements to facilities.”
The Full Spectrum
Although reduction in solar heat gain and cooling costs are the primary benefits of window film, there are other advantages as well. The near-complete rejection of UV rays can decrease fading of furniture and mitigate the harmful effects UV rays have on building occupants.
“All quality window films reject between 95 and 99 percent of ultraviolet energy,” says Smith. “UV is not considered part of heat gain, but it can be destructive as well as cause long-term exposure problems to individuals.”
Window film also reduces the amount of visible light that comes through a window, which can help reduce glare on computer screens and other reflective surfaces. The amount of visible light transmitted is part of the SHGC, so the amount of visible light rejected will depend on how much near-infrared radiation is rejected. Visible light transmission can range from 3 to 5 percent for privacy films, and up to 88 percent for clear UV films, Smith says.
While blinds or shades can also be used to limit visible light, window film can achieve this goal while creating a more uniform look on the outside of the building. It also preserves the view for occupants.
“The building occupant is looking for a view and is paying for a view,” Mannix says. “You want the space to be comfortable while maintaining that view.”
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