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.
The solution to overheating through windows is to specify heat-blocking solar control glass or applied window film, though even the best solar control glass performs no better than the best applied window film. Solar control glass can be selected for optimum energy performance in reference to the geographic orientation of any given building or section of a building. However, even in new construction, often the cost of solar control glass exceeds the cost of standard glass to which a solar control film is later applied.
For existing buildings experiencing problems from heat through windows, certainly 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. Less expensive is keeping existing frames and replacing only the glass. In either case, building managers understandably may be reluctant to replace existing windows or glass whose performance is generally adequate though not optimum in the case of blocking unwanted heat.
For all existing glass and in much new construction, applied window film is the least expensive and preferred solution to mitigate the impact of too much solar heat entering windows. The good news is conventional tinted and reflective applied window films successfully block some 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. Many of these films are highly reflective in daylight, giving them a mirror like appearance when viewed externally. In artificial light and at night, internally, reflective films appear mirrored. In the case of retail establishments, visible light is reduced inside the store and shoppers outside cannot clearly see inside.
Most conventional window films transmit less than 34 percent of visible light, a good 36 percent less than the 70 percent necessary to be undetected by the naked eye. The result is building interiors are correspondingly darkened, often requiring the use of increased illumination. This may lead to higher electricity consumption that may increase inside temperatures requiring more air-conditioning. Increased utility costs defeat the major benefit of the film - cost savings.
The best solution to overheating - clear, spectrally-selective film
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 does a film transmit? Most so-called spectrally selective films transmit no more than 54 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.
The following points should be considered when evaluating spectrally selective vs. conventional window film:
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 34 percent of visible light and correspondingly appear unclear. However, even a film with light transmission 36 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 in so doing possesses a clear appearance.
Blocking heat -- Most conventional tinted films transmit over 65 percent of solar energy giving them an unacceptable shading coefficient of over 0.70. (The lower the shading coefficient the lower the solar heat gain.) Some reflective films with a shading coefficient as low as 0.28 transmit as little as 15 percent of visible light. When considering both heat rejection and light transmission, spectrally selective films out perform conventional competitors.
Mitigating heat loss -- Both conventional and spectrally selective window films are designed primarily to block near infrared or solar heat. However, both conventional and spectrally selective window films will enhance the ability of existing glass to insulate against heat loss by as much as 15 percent.
Application -- 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.
Care and maintenance -- 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.
Aesthetics -- Conventional dark and reflective window film changes the appearance of existing glass and therefore the external appearance of a building. 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.
Price -- The price of dark, tinted and reflective window film ranges from $4 to $6 per installed square foot. 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.
Payback -- Less expensive conventional window films have a shorter payback compared to more expensive spectrally selective films. However, when you add on the cost of extra energy used for lighting and HVAC operation due to conventional films inability to transmit sufficient visible light, the payback 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 always improving - averaging less than 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.
Houston-based V-Kool, Inc. is a sales and marketing distribution company of spectrally selective applied films for architectural, automotive and specialized vehicular applications. For information visit www.v-kool-usa.com.>