A window film is a thin, transparent sheet that can be applied to the interior of a window to change its heat- and light-transmitting characteristics. Films reduce cooling loads, improve shatter resistance, block up to 99 percent of ultraviolet (UV) radiation, and reduce glare. Temperatures near the windows are reduced as well, which can increase occupant comfort.
For retrofit applications, many of the benefits of window materials that control solar heat gain are available by applying window films to existing glazings (Figure 1). Typical films have a total thickness of 0.001 to 0.004 inches. They are made with a variety of adhesives and can be applied on-site to single- or double-glazed windows, usually to the inner surface, facing the room. Some of the early window film products suffered from problems with film fading, color shift, installation difficulties, and poor adhesive performance. Those difficulties have largely been solved with the newest products and application techniques. Payback periods of less than three years have been reported.
Currently, there are a variety of window film products on the market. Many of these films are now rated in combination with windows using two standard metrics that are also used for windows on their own.
Solar heat gain coefficient (SHGC). SHGC measures how well a window/film combination blocks heat from sunlight. The SHGC is the amount of the heat from the sun that enters through the window/film combination, expressed as a number between 0 and 1. The lower a combination’s SHGC, the less solar heat it transmits.
Visible transmittance. Visible transmittance, or Tvis, measures the percentage of visible light that makes it through a window/film combination. It is expressed as a number between 0 and 1. The higher a combination’s Tvis, the more light it transmits.
The only nationally accepted certification for window/film combinations comes from the National Fenestration Rating Council (NFRC), which released a voluntary rating and labeling system for window films in late 2006. The NFRC provides both net SHGC and visible transmittance measurements for window/film combinations (for more information, see its brochure, “NFRC’s Window Film Certification and Labeling Program” [PDF]). Films are rated in combination with single- and double-pane clear and gray-tinted windows for commercial applications. The measurements can be found in the NFRC’s Directory of labeled fenestration products. You can search the directory for a window film by manufacturer and then see the ratings for that film on various glazing systems. Unlike the NFRC film rating label, the directory provides performance information for windows with and without a film applied.
Standard. Standard window film is a low-cost method of reducing cooling loads, with the side benefits of glare reduction, increased shatter resistance, and absorption of UV radiation. However, most of these products reduce daylight as much as or more than they reduce solar heat gain.
Spectrally selective. Spectrally selective window films reduce solar heat gain effectively while transmitting more of the sun’s visible light than standard films do. Table 1 presents typical values for various window films. Although spectrally selective films cost about twice as much as standard films, they make it possible to capture additional energy savings through the use of daylight-dimmable lighting systems.
Other window coverings. Some of the benefits of window films can be obtained by more conventional means such as shutters, shades, and draperies. However, shutters hide the beauty of the window and darken a room when closed. Shades can also block much of the outside view and reduce the ability to use daylight. Draperies, although they add design appeal to a room, aren’t much help in controlling energy loss, even when closed.
Review occupant comfort. Are occupants complaining of overheating or glare? Are they closing shades and turning on lights? If you answered yes to any of these, there’s a good chance that window films can help your facility save energy.
Estimate the savings potential. A computer simulation is usually necessary to calculate the potential energy savings from window film installation. This is because the film affects both HVAC and lighting loads—and, in turn, a reduction in lighting loads has an additional impact on HVAC. If using daylighting is feasible, consider spectrally selective films. COMFEN, a free simulation tool developed by Lawrence Berkeley National Laboratory, can estimate savings given the net performance metrics of a window/film combination as well as other variables such as geographic location and energy costs.
Pick a product with a long warranty from a reputable manufacturer. As with any product, window film quality varies among manufacturers. Films generally last for 5 to 15 years, depending on both the quality of the product and the environment in which it is installed. Ask the film manufacturers about the expected lifetime of their product in a particular application. Warranties are typically for 5 to 10 years, although some manufacturers offer lifetime warranties under specific conditions.
Avoid using films in some situations. There are reports of windows cracking due to heat build-up when dark film is used on nonstrengthened glass that has a high solar exposure. Films also should not be used:
Manufacturers are working to make films with greater UV-blocking capability to protect furnishings and to make a film that will be durable enough to be applied to the exterior of a window and still maintain its performance characteristics for at least five years. Also, the number of products covered by the NFRC directory will continue to expand as additional manufacturers have their products certified.