Window Film

A window film is a thin, transparent sheet that can be applied to the interior of a window to change the 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. It’s important to note that films are most suited to buildings in hot climates, where they can reduce cooling loads; in cold climates, however, the reduced solar heat gain can increase heating loads.

In retrofit applications, you can apply window films to existing glazings to obtain many of the benefits of window materials that control solar heat gain (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 that faces the room. Some early window film products suffered from fading, color shift, installation difficulties, and poor adhesive performance, but those difficulties have largely been resolved with the newest products and application techniques. Payback periods of less than three years have been reported.

Figure 1: Window films help control solar impact
Window films help block infrared radiation, the major solar heat transfer mechanism. They also block ultraviolet (UV) radiation that causes fabrics and other materials to fade. The amount of visible and infrared light transmitted varies from product to product, but most films block the majority of UV light.
Figure 1: Window films help control solar impact
What are the options?
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A variety of window film products are on the market. Many are rated in combination with windows using two standard metrics that are also used for windows without films.

Performance metrics

Solar heat gain coefficient (SHGC) measures how well a window/film combination blocks heat from sunlight. The SHGC is the amount of solar heat that enters through the window/film combination, expressed as a number from 0 to 1. The lower the SHGC, the less solar heat is transmitted.

Visible transmittance (Tvis) measures the percentage of visible light that makes it through a window/film combination. It is also expressed as a number from 0 to 1; the higher the Tvis, the more light is transmitted.

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 2006. It provides net SHGC and Tvis measurements for window/film combinations (see 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 are published in the NFRC 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.

Window film types

Standard window film is a low-cost method of reducing cooling loads that offers the side benefits of glare reduction, increased shatter resistance, and UV absorption. However, most standard products reduce daylight as much as or more than they reduce solar heat gain.

Spectrally selective window films reduce solar heat gain effectively while transmitting more of the sun’s visible light than do standard films (Table 1). Although spectrally selective films cost about twice as much as standard films, they make it possible to capture additional energy savings through daylight-dimmable lighting systems.

Table 1: Effects of window films
Numerous window film products are available with a wide range of specifications. We chose a sample of each type that can modify the performance of clear glass to show here. The lower the SHGC, the less solar heat the window allows through; the higher the Tvis, the more light is transmitted.

Other window coverings, such as shutters, shades, and draperies, can provide some of the benefits of window films through more conventional means. However, shutters hide the beauty of the window and darken a room; shades can also block much of the outside view and reduce the ability to use daylight; and draperies, which may add design appeal to a room, aren’t much help in controlling energy loss, even when closed.

How to make the best choice
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Review occupant comfort. Are occupants complaining of overheating or glare? Are they closing shades and turning on lights? If so, 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.

Look for long warranties and reputable manufacturers. As with any product, window film quality varies by manufacturer. Films generally last 5 to 15 years, depending on both the quality of the product and the environment in which it is installed. Ask manufacturers about the expected lifetime of their products in a particular application. Warranties typically run 5 to 10 years, although some manufacturers offer lifetime warranties under certain conditions.

Avoid film for 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. Film also should not be used on:

  • Single panes larger than 100 square feet or double panes larger than 40 square feet
  • Clear glass thicker than three-eighths inch or tinted glass thicker than one-quarter inch
  • Glass with a nonstandard composition or surface, such as reflective, laminated, wired, patterned, or textured glass
  • Glass in certain kinds of framing such as concrete, solid aluminum, or solid steel
  • Cracked or otherwise damaged glass
What’s on the horizon?
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Manufacturers are working to make films with greater UV-blocking capability to protect furnishings. They are also exploring exterior applications that maintain performance characteristics for at least five years. The number of products covered by the NFRC Directory will continue to expand as additional manufacturers have their products certified.

In 2015, researchers at the University of Texas at Austin produced a “smart” window film that blocks heat from the sun when it’s hot but allows heat to pass through when it’s cold. This is achieved by embedding indium tin oxide nanocrystals in glass imbued with niobium oxide, which results in a film that can transmit or block light depending on the electric potential applied. Previous “electrochromic” materials took hours to change mode, but this new smart coating can transition in minutes—bringing the technology much closer to commercialization. This material is undergoing method testing for low-cost manufacturing.

Who are the manufacturers?
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