Window Films: Spectrally Selective versus Conventional Applied (Energy Engineering)

Abstract

Compared to conventional applied window film, high light-transmitting spectrally selective film cost-effectively blocks unwanted solar energy from entering windows and reduces air conditioning costs without darkening building interiors, impeding the ability to see through existing glass, or changing the external appearance of a building.

According to the California Energy Commission, as much as 40% of a building’s cooling requirements is a function of heat entering through existing glass. Stopping heat at the window is the most effective means of lowering temperatures and reducing heating, ventilation and air conditioning (HVAC) operating costs. 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 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, the cost of solar control glass often exceeds the cost of standard glass to which a solar control film is later applied.

For existing buildings experiencing problems from heat through windows, 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 may be understandably 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. Conventional dark and reflective applied window films successfully block a significant amount of solar heat, thereby reducing the use of HVAC systems.

Unfortunately, the 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. In the case of retail establishments, visible light is reduced inside the store and shoppers outside cannot see clearly inside.

Most conventional window films transmit less than 35% of visible light, a good 35% less than the 70% necessary to be undetected by the naked eye. The result is that building interiors are correspondingly darkened, often requiring the use of increased illumination. This leads 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 the film transmits. Most so-called spectrally selective films transmit no more than 54% 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 table shows how different kinds of glass and applied films transmit light and heat.

Building management should consider the following points when evaluating spectrally selective vs. conventional window films.

Table 1 Window film performance

Type of glass or applied film Percentage of daylight through glass Percentage of solar energy through glass Shading coefficient” Luminous efficacy constant Percentage of light reflectance interior/exterior
1/i” clear glass 89 77 0.96 0.93 7/7
1/i” clear glass with tinted film 37 64 0.74 0.50 6/6
1/i clear glass with reflective film 37 44 0.51 0.73 18/28
1/i” 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.

bLuminous efficacy 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.

How do they Compare in 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 35% of visible light and correspondingly appear unclear. Spectrally selective film, which blocks heat equivalent to the darkest films, transmits 70% of the visible light and in so doing possesses a clear appearance (Table 1).

How do they Compare in Blocking Heat?

Most conventional tinted films transmit over 65% of solar energy, giving them an unacceptable shading coefficient of over 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% of the visible light. When considering both heat rejection and light transmission, spectrally selective films out perform conventional competitors.

How do they Compare in Mitigating Heat Loss in Cold Weather?

Both conventional and spectrally selective window films are designed 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%.

How do they Compare in Applicability 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 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. For further information on the use of window film on insulating glass consult AIMCAL, Ft. Mill, SC (www. aimcal.com).

How do they Compare in Requiring 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.

How do they Compare in Price?

The price of dark, tinted, and reflective window film ranges from 4 to 6 dollars 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 per square foot installed. Installed prices are volume dependent; on larger projects such superior performing films may be installed for less.

How do they Compare Aesthetically?

Conventional dark and reflective window film changes the appearance of existing glass and therefore the external appearance of a building. Clear, spectrally Window Films: Spectrally Selective versus Conventional Applied 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.

How do they Compare in Payback?

Less expensive, conventional window films have a shorter payback compared to more expensive, spectrally selective films. However, it’s not that simple. It is necessary to add on the cost of extra energy used for lighting due to the inability of conventional film to transmit sufficient visible light. Also, because extra lighting generates additional heat, the use of conventional window film may also increase air conditioning cost.

In reality, 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.

Use of Spectrally Selective Applied Window Film at Stanford University

Spectrally Selective Window Film Saves Energy at Stanford University.

While recent summers will be remembered for actual and threatened energy blackouts in California, Stanford University is doing its part to reduce energy use, thanks to the ongoing energy conservation program. Among many measures taken to improve energy efficiency at Stanford is the installation of clear, spectrally selective applied window film in 20 academic and administrative buildings on the 8,000 acre campus south of San Francisco.

At Stanford, the least expensive option to dramatically reduce unwanted solar heat and improve the performance of existing windows is applied window film. Scott Gould, Stanford’s energy engineer, contends that since the 80s, window film performance has improved and that films look better and last longer. However, it was clear to decision-makers at Stanford that not all windows films are alike.

Not any film would do.

According to Alan Cummings, associate director of Facilities Operations, the primary reason for window film at Stanford is heat loading and occupant comfort. In collaboration with campus architect David Neuman, Cummings reviewed a variety of applied film products. Their primary objective was to select a window film with high light level transmission and heat load reduction. Equally important was the need for a window film that would not appear reflective. According to Neuman, the traditional architecture of the campus precludes using reflective glass, even on the newest buildings.

Conventional mirrored and tinted window films do prevent some solar heat loading, but cannot transmit high levels of light. Spectrally selective film freely transmits visible daylighting while blocking the near infrared and UV portions of the sun’s spectrum. Tinted films may reduce heat gain but darken building interiors; spectrally selective film is virtually clear and so doesn’t change the color of existing glass.

Over the past five years, spectrally selective applied film has been applied to selected south-, west-, and east-facing facades on 20 Stanford buildings totaling 120,000 ft2. They include the Stanford Law School and the Green Earth Sciences Building. The film’s energy payback, through lowered air-conditioning bills, is from three to four years, depending on the building, electricity rate, and weather.

The most recent film installation at Stanford took place at Encina Hall, a renovated administration building that was originally constructed as a dorm in 1891 and completely renovated in 1998. Some 6,212 ft2 of film was applied in June, 2003.

According to a before and after energy audit conducted by V-Kool, Inc., the building has a British thermal unit meter reading (Btu/ft2/h.) of 225 and 5.71 h a day of peak load. Daily air conditioning requirements to remove heat without the spectrally selected film amounted to 665.57 A/C tons to remove heat at a cost of AC at $66.56 per day. Daily air conditioning requirements with the film installed are 339.44 A/C tons to remove heat at a cost of AC at $33.94 per day.

Energy savings on the building with the film consist of:

1. $32.61 daily in A/C savings;

2. $978.39 in monthly A/C savings;

3. $4,891.95 in annual A/C savings.

Approximate project cost: $43,000. Return on investment: nine years or less, gave increase in the cost of electricity.

The window film installation program at Stanford University is an example of a long-term commitment to energy conservation on a campus with some buildings over 100 years old. To our knowledge, there are more buildings equipped with spectrally selective window film at Stanford University than at any other single institution in the country.

A case in point is the Los Angeles Department of Water and Power’s (LADWP) rebate program for window film. It is based on a film’s luminous efficacy 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 square foot rebate from LADWP, a spectrally selective film that blocks less heat but lets in more light receives a higher rebate of 85 cents per square foot. Only spectrally selective films with luminous efficacy constants over 1.0 receive the higher rebate.

How do they Compare in 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.

REAL-LIFE INSTALLATIONS OF SPECTRALLY SELECTIVE WINDOW FILM

Both company-owned and franchise properties of the following retailers use spectrally selective window film in selected establishments: Hallmark Cards, Calico Corners, Public Storage, Albertson’s, Esprit, McDonalds, Exxon, and Quik Trip convenience stores. Spectrally selective window film is saving energy at the Eldorado Country Club in Palm Springs, the Ontario, CA, convention center, and in such landmark buildings as the former headquarters of Montgomery Ward in Chicago and the headquarters of the American Institute of Architects in Washington, DC.

CONCLUSION

The universal applicability of spectrally selective applied window film makes it particularly cost effective for institutional, commercial, and residential structures in need of solutions to one or more of the following problems: solar overheating; reduction in HVAC size, operation, and cost; reduction in discomfort to building occupants and visitors due to high temperatures; increasing productivity of building occupants through maximizing natural light; utilization of floor space adjacent to windows and fixed glass; some increase in insulation against heat loss in winter; reduction of some glare while maintaining high levels of natural light; reducing internal temperature swings that can damage expensive interior landscaping; increasing a building’s energy efficiency without compromising its historic and aesthetic qualities; mitigating the impact from ultraviolet radiation (UV) induced fading of furniture and window treatments; reducing exposure to cancer-causing UV, and increasing the resistance of existing glass to wind-blown debris, earthquake stress, explosions, and forced entry.

Of course, film manufacturers should and will continue research and development of applied films that will block even greater amounts of heat while transmitting high levels of visible light. Other enhancements in applied window film that may one day become available include increased durability, strength, and resistance to environmental and climatic stress, resulting in even greater film life expectancy.

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