Comparing the Energy Conservation Capabilities of Spectrally Selective and Conventional Applied Window Film
- By Marty Watts
- November 1st, 2003
According to the California Energy Commission, 30 percent of a campus 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 costs. In new construction, this can mean the need for smaller and less expensive HVAC systems.
The solution to over-heating through windows is to specify solar control glass or applied window film, even though the best of either type performs about the same. Solar control glass can be selected for optimum energy performance in reference to the geographic orientation of any given building or building section. However, even in new construction, often the cost of solar control glass exceeds the cost of standard insulating glass to which a solar control film is later applied.
For existing campus buildings, the most expensive option is to replace existing glass and frames with a new window system designed to block heat and deal with a building's energy performance needs. A more cost-effective solution is to keep existing frames and only replace the glass.
For existing glass and in much new construction, applied window film is the least expensive and more preferred. The good news is conventional dark and reflective applied window films successfully block a significant amount of solar heat, thereby reducing the use of HVAC systems.
The bad news is that these same films reduce a significant percentage of visible light through the glass. Most of these films are highly reflective in daylight, giving them a mirror-like appearance when viewed externally. In artificial light and at night, internally they appear mirrored.
Most conventional window films transmit less than 58 percent of visible light, a good 10 percent less than the 70 percent necessary to be undetected by the naked eye. This means building interiors are correspondingly darkened, and need increased illumination. This leads to higher electricity consumption that may increase inside temperatures, so more airconditioning is required. Increased utility costs defeat the major benefit of the film -- cost savings.
Clear spectrally selective applied window 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.
While some manufacturers call their films spectrally selective, the definitive test is how much visible light film transmits. Most so-called spectrally selective films transmit no more than 58 percent of visible light. If a window film looks tinted and not clear, it is not optimally selective in the all-important category of visible light transmission.
Follow the following nine points when evaluating spectrally selective versus conventional window film.
1. Clarity: The ideal film would be totally clear, yet able to significantly block unwanted solar heat and reduce glare. Most dark and reflective films transmit less than 58 percent of visible light and correspondingly appear unclear. Even a film with light transmission 12 percent below normal does not achieve maximum heat rejection, as darker reflective films block more heat. Spectrally selective film, which blocks heat equivalent to the darkest films, transmits 70 percent of the visible light and has a clear appearance.
2. Heat blockage: Most conventional tinted films transmit more than 65 percent of solar energy, giving them an unacceptable shading coefficient of more than 0.70. (The lower the shading coefficient, the lower the solar heat gain.) With a shading coefficient as low as 0.51, reflective films block more heat, but many transmit as little as 15 percent of the visible light. When considering both heat rejection and light transmission, spectrally selective films outperform conventional competitors.
3. Reduction of heat loss in cold weather: Both conventional and spectrally selective window films are designed primarily to block near infrared or solar heat. However, both will also enhance the ability of existing glass to insulate against heat loss by as much as 15 percent. This means window film can help save building energy during the heating season.
4. Application to different types of glass: 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 auspicious of the Association of Industrial Metallizers, Coaters and Laminators (AIMCAL), applied window film that is 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. For further information on the use of window film on insulating glass, consult AIMCAL, Ft. Mill, SC (www.aimcal.com).
5. Care: The best-applied films require no special care. They can be cleaned just like the surface of glass. Use no abrasives, just soap and water.
6. Price: The price of dark, tinted and reflective window film ranges from $4 to $6 per installed sq. ft. The best spectrally selective applied window film ranges in price from approximately $9 to $12 per sq. ft. installed. Installed prices are volume-dependent, so on larger projects, such superior performing films may be installed for less.
7. Aesthetics: Conventional dark and reflective window film changes the appearance of existing glass and the external appearance of a campus building. Clear spectrally selective film does not change the appearance of existing glass, allowing application on the entire building, or just a few windows to deal with a localized over-heating problem. For limited applications, both films are competitive in price.
8. Long-term investment: Less expensive conventional window films have a shorter payback than more expensive spectrally selective films. However, when you add on the cost of extra energy used for lighting and HVAC operation, because of conventional film's inability to transmit sufficient visible light, the pay-back for conventional film and spectrally selective film becomes comparable. Given rising electricity and natural gas rates, the rate of payback for spectrally selective film is improving, averaging about four years. Case in point, the Los Angeles Department of Water and Power's (LADWP) rebate program for window film is based on a film's luminous efficiency constant, a measurement of its ability to simultaneously block heat and transmit light. While a very reflective film that blocks more heat than a spectrally selective film earns a 55 cent per sq. ft. rebate from LADWP, a spectrally selective film that blocks less heat but lets in more light receives a higher rebate of 85 cents per sq. ft. Only spectrally selective films with luminous efficiency constants over 1.0 receive the higher rebate.
9. 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 and the installer.
Marty Watts is president & CEO of V-Kool, Inc., in Houston, a sales and marketing distribution company of spectrally selective applied films for multiple types of applications.