According to the California Energy Commission, 30 percent of a building’s cooling requirements are a function of heat entering through existing glass. Stopping heat at the window is the most effective means of lowering temperatures and reducing HVAC operating cost. In new construction, reducing heat at the window can mean the need for smaller and less expensive HVAC systems.One excellent solution to overheating is to use clear, spectrally-selective applied window film. This film offers the best ratio of visible light transmission to heat rejection. Spectrally-selective refers to the ability of the film to select or let in desirable daylight, while blocking out undesirable heat. Consider the following :Clarity: The ideal film would be totally clear, yet able to significantly block unwanted solar heat and reduce glare. Spectrally selective film, which blocks heat equivalent to the darkest films, transmits 70 percent of the visible light, and in so doing possesses a clear appearance.Applicability: Both conventional and spectrally selective films can be applied to single pane and insulating fixed glass, windows, and doors. Always identify existing glass and follow the advice of a qualified film installer.According to tests conducted by independent laboratories under the auspices of the Association of Industrial Metallizers, Coaters and Laminators (AIMCAL), applied window film properly installed on insulating glass does not cause seal failure. Accordingly, most window film manufacturers offer an insulating glass warranty in the event of seal failure. Special care: The best applied films require no special care. They can be cleaned just like the surface of glass using no abrasives, just soap and water.Price: Depending on the particulars of the installation and the geographic area, the best spectrally-selective applied window film ranges in price from approximately $9 to $12 a square foot installed. Installed prices are volume dependent; therefore, on larger projects such superior performing films may be installed for less.Aesthetics: Clear spectrally-selective film does not change the appearance of existing glass, allowing its application on the entire building or on as few windows as necessary to deal with a localized overheating problem. For limited applications, spectrally-selective film is competitive in price with conventional film. Given rising electricity and natural gas rates (and the ability of spectrally-selective film to reduce lighting and HVAC needs), the rate of payback for spectrally-selective film is always improving – averaging about four years.Guarantees: The best applied films are guaranteed not to peel, discolor, blister, bubble, or demetalize for at least 10 years on a commercial installation. Look for a guarantee from the manufacturer in addition to any by the installer.Resources: For more information on conventional and spectrally-selective window film, contact the International Window Film Association (www.iwfa.com), Martinsville, VA, and the Association of Industrial Metallizers, Coaters and Laminators (www.aimcal.com), Ft. Mill, SC.Marty Watts is president and CEO at Houston-based V-Kool Inc. (www.v-kool-usa.com) or (800) 217-7046.