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.
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.