Window film delivers energy savings without compromising light transmittance

The latest window film technology delivers energy savings potential without compromising natural light transmittance
August 11, 2010

Window film has become one of the most cost-effective methods for cutting energy usage in existing buildings, especially in warm, sunny climates like Florida, California, and Arizona. By blocking large amounts of the sun's infrared rays, these micro-thin films can reduce heat gain, thereby lowering air-conditioning costs.

Window film also reduces sun glare, balances internal air temperature better (fewer hot and cold spots), and virtually eliminates ultraviolet rays, which can cause carpet and other interior surfaces to fade. Moreover, if properly adhered to the window frame, window films can provide protection against flying glass fragments in the event of a blast or natural disaster.

One common complaint among Building Teams, owners, and occupants is that traditional window film drastically cuts the amount of natural light entering the building — some films block as much as 88% of daylight. The general rule is the better a product's heat gain performance, the less amount of visible light transmitted through the glass, because heavier levels of heat-absorbing metal are required in the film.

"The window film industry has always been able to block UV and varying degrees of infrared or heat, but they've done that at the cost of darkening the windows or making them more reflective," says Paul Murphy, owner of Bay Area Window Film, a Foster City, Calif.-based window film contractor.

Murphy says newer film technology, called "spectrally selective" film, lessens this tradeoff between performance and visibility. Through the use of a process called sputter deposition, manufacturers are now able to deposit a wide range of alloys and exotic metals — such as titanium and silver — onto window film to block more of the near-infrared, or heat portion, of the spectrum and less of the visible light.

"[Manufacturers] take the entire solar spectrum and divide it into selected parts — UV, visible light, and infrared," says Darrell Smith, executive director of the International Window Film Association and director of the window film committee at The Association of Industrial Metallizers, Coaters, and Laminators. "They can engineer each wavelength to reflect or absorb light, and how much."

Several manufacturers, including V-Kool, Southwall Technologies, 3M, Bekaert Specialty Films, and CP Films, have developed spectrally selective films. V-Kool's V-Kool 70 film, for instance, allows in 74% of visible light, while rejecting 94% of infrared and 99% of UV rays, which results in 55% less solar heat. Traditional films can cut solar heat by as much as 80%, but transmit much less visible light, says Smith.

 
Stanford University’s Encina Hall was recently retrofitted with more than 6,200 square feet of spectrally selective film. The 112-year-old building is one of 18 academic and administrative structures retrofitted with spectrally selective film on selected south-, west-, and east-facing façades as part of an on-going energy conservation program.


"Even though the absolute heat rejected might not be as high using spectrally selective films, the appearance is much more pleasing, but you pay a premium for it," adds Smith. He says the spectrally sensitive film can cost anywhere from $6-12 per square foot, compared to $3 per square foot for standard window film.




"The big push today in green design is to take advantage of natural light as much as possible because it cuts down on lighting costs," says Thomas F. O'Connor, FAIA, VP and director of building technology with Detroit-based SmithGroup. "Having a film that will transmit more light and also stop infrared and UV is ideal."

One of the largest installations of spectrally selective film is taking place at Stanford (Calif.) University, where 18 academic and administrative buildings have been retrofitted with V-Kool film on selected south-, west-, and east-facing façades as part of an on-going energy conservation program.

David Neuman, campus architect, says the film was selected based on campus-wide standards in terms of reflectivity, color, opacity, and thermal performance.

"I don't mind a little color, but I did not want the film to be too reflective, particularly in older buildings, where we're concerned about the character of the glass," says Neuman.

The film allows more than 70% light transmittance and has a shading coefficient of 0.51, which means building occupants notice a "minimal tint," says Neuman. "If we put new performance low-e glass in the same situation, you'd probably get about the same amount of reflectivity and transparency."

The film also blocks 99% of UV light that causes fading damage and has a scratch-resistant coating.

The university's most recent installation was at Encina Hall, a renovated administration building that was originally constructed as a dorm in 1891 and completely renovated in 1998. Bay Area Window Film applied more than 6,200 square feet of spectrally selective film in June 2003.

 
V-Kool spectrally selective window film allows more than 70% of light transmittance, while blocking the majority of infrared and ultraviolet rays.


According to an energy audit conducted by V-Kool, the retrofit cut the building's air-conditioning load requirement nearly in half, from 665.57 AC tons per day to 339.44 AC tons per day. That results in average energy savings of $32.61 per day and projected annual savings of $4,891.95.




At that rate, the $43,000 project would pay for itself in approximately nine years.

"With products like V-Kool, you're going to have longer paybacks," says Murphy. "But for a university, with all these historic buildings, it makes sense."

Light and heat transmittance characteristics of glass and applied films

Type of glass or applied film % of daylight through glass % of solar energy through glass shading coefficient* luminous efficiency constant** % of visible light reflectance interior/exterior
1/4-in. clear glass 89 77 0.96 0.93 7/7
1/4-in. clear glass with tinted film 37 64 0.74 0.50 6/6
1/4-in. clear glass with reflective film 37 44 0.51 0.73 18/28
1/4-in. clear glass with clear spectrally selective film 70 45 0.51 1.37 8/8
* The lower the shading coefficient, the lower the solar heat gain.
** Luminous efficiency constant, a measurement of a window glass or film's ability to simultaneously block heat and transmit light. (Visible light divided by the shading coefficient). The higher the number the more efficiently the glass or film blocks heat and transmits light.





Suppliers of spectrally selective window film
3M: www.3m.com Bekaert Specialty Films: www.bsf.com CP Films Inc.: www.cpfilms.com
Film Technologies International: www.filmtechnologies.com Southwall Technologies: www.southwall.com V-Kool: www.v-kool-usa.com