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 radiation, and reduce glare. Temperatures near the windows are reduced as well, which increases 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. To help distinguish them, performance metrics have been devised that describe the benefits and limitations of each type.

Window Film Performance Metrics

Solar heat gain coefficient (SHGC). SHGC measures how well a window film blocks heat from sunlight. The SHGC is the amount of the heat from the sun that enters through a window, expressed as a number between 0 and 1. The lower a window film's SHGC, the less solar heat it transmits.

Visible transmittance (Tvis). Tvis measures the percentage of visible light that makes it through a window film. It is expressed as a number between 0 and 1. The lower a window film's Tvis, the less light it transmits.

Light-to-solar-gain ratio (LSG). LSG measures the ratio of Tvis to SHGC. A ratio greater than one indicates that the window film transmits more light than heat. Windows films with low LSGs are more often used in passive solar heating applications, whereas window films with high LSGs are used to help prevent heat gain in a building.

Types of Window Films

Standard films. The standard window film offers a low-cost method of reducing cooling loads, with the side benefits of glare reduction, increased shatter resistance, and absorption of ultraviolet (UV) radiation. However, most of these products reduce daylight as much as or more than they reduce solar heat gain. These films have lower LSGs.

Spectrally selective films. 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. These films have higher LSGs.

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, there's a good chance that window films can help your facility save energy.

Estimate the savings potential. A computer simulation will usually be necessary to calculate the potential energy savings from window film installation. This is because the film affects both HVAC and lighting loads, and lighting loads also have an impact on HVAC. If using daylighting is feasible, consider spectrally selective films.

Pick a product with a long warranty from a reputable manufacturer. 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 film manufacturers about the expected lifetime of their product in a particular application. Warranties are typically 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:

• On single window panes larger than 100 square feet or on double panes larger than 40 square feet
• On clear glass thicker than three-eighths inch or tinted glass thicker than one-quarter inch
• On glass with a nonstandard composition or surface, such as reflective, laminated, wired, patterned, or textured glass
• On cracked or otherwise damaged glass
• On glass in certain kinds of framing such as concrete, solid aluminum, or solid steel

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.