Energy Advisor
Your guide to energy products
for commercial buildings
|
Energy Advisor Your guide to energy products |
Building Shell: Window Film
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 solar-control glazings are available by applying window films to existing glazing areas (see 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.
Visibility through the display windows of this Esprit showroom remained high after the installation of spectrally selective window film.
Source: Platts
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.
Spectrally selective films. Spectrally selective window films reduce solar heat gain effectively while transmitting more of the sun's visible light than do standard films. The relative amounts of daylight transmitted by spectrally selective and standard films are shown in Figure 2. 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.
Spectrally selective films such as SOLIS transmit a high proportion of visible light while limiting the amount of solar heat gain.
Source: Platts
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 in certain situations.
Avoid using films in certain situations. There are reports of windows cracking due to heat build-up when dark film is used on nonstrengthened glass with a high solar exposure. Films also should not be used in the following applications:
• On single window panes larger than 100 square feet or on double panes larger than 40 square feet;
• On clear glass thicker than 3/8 inch or tinted glass thicker than 1/4 inch;
• On glass with a nonstandard composition or surface, such as reflective, laminated, wired, patterned, or textured glass;
• On cracked or otherwise damaged glass; or
• 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 UV-sensitive furnishings and to make a film that will be durable enough to be applied to the exterior of a window and maintain its performance characteristics for at least five years.
Copyright © 2004 - Platts, a Division of The McGraw-Hill Companies, Inc.
