Window films can conjure up images of hazy purple film bubbling off of automobile glazing. In today’s buildings, however, they represent both an established and evolving technology that can significantly reduce solar heat gain and glare. By giving us daylighting and views, windows provide immeasurable boosts to our productivity and well-being. However, they also increase HVAC loads, which account for 39 percent of the energy that buildings consume, according to the U.S. Department of Energy.

For older buildings with windows installed before the days of low-emissivity (low-E) and insulated glass, applied aftermarket films may be an effective alternative to window replacement, with paybacks as short as three years—and with occupants none the wiser.

Shiny to Subtle

“The first window films looked like tin foil,” says Billy Pettit, senior technical service engineer at 3M Corp. “They did a great job of keeping the heat out, but they had a mirrorlike appearance.” Early films, introduced as early as the 1960s, blocked the entire solar spectrum of infrared, visible, and ultraviolet light without discrimination; as a result, if the films chosen were too dark, occupants often had to turn on artificial lights even on the clearest days.

As aesthetics and performance demanded new technology, film capabilities and product offerings expanded; to date, the National Fenestration Rating Council’s Certified Products Directory has more than 1,200 registered window films. In lieu of stainless steel and titanium coatings first used in the late 20th century, applied films now employ proprietary concoctions comprising more than 200 layers of polymeric material. An adhesive layer on the glass-side face and a hard, scratch-resistant coating on the outer face complete the sandwich of materials, which total just 2 to 14 mils thick—thinner than a sheet of paper.

Spectrally selective films, which have coatings designed to reflect or absorb specific wavelengths in the solar spectrum, have reduced the trade-off between reflectance and visibility. Low-E films, which reflect long-wave infrared energy and suppress thermal radiation, can also increase a window unit’s insulating capabilities. “Now you can customize your film needs to the end-use of the building,” says Darrell Smith, executive director of the International Window Film Association (IWFA). “You can go from dark or shiny to clear to the naked eye, and with almost as much heat rejection.”

Two measures of a film’s energy performance are visible transmittance (VT) and solar heat gain coefficient (SHGC), which range from a unitless measurement that is between zero and 1. A higher VT value indicates greater amount of visible light passing through the film, while a lower SHGC means more solar radiation is rejected by the film. Mid-range products with VT and SHGC values around 0.5 are “ideal for northern climates,” Smith says, because they block some solar energy in the summer but still allow some “passive solar heat in the winter.” Films where VT greatly exceeds the SHGC are suitable for hot climates. Mid-range films can cost $6 to $8 per square foot installed, Smith says, while high-end films with VT exceeding 0.6 and SHGC about 0.4 or higher can cost $8 to $14 per square foot installed.

Clear Choice

At 111 West Jackson, a 24-story commercial office building in Chicago’s financial district, the existing window film on its curtainwall exterior “made the interiors look really dark,” says Greg Prather, senior vice president at Jones Lang LaSalle (JLL), which began renovating the 1960s building last spring. “We were looking for a window film that would give some energy efficiency and cut down [on glare] without making the spaces look dark.”

Working with local installer National Security & Window Filming, JLL is applying films with a VT of 0.60 and SHGC of 0.47 on the building’s original single-pane glass on the east, west, and south elevations. “It’s much lighter than what was up there,” Prather says. “It has really cut down on the amount of glare and heat inside.”

The ability of films to provide UV protection without affecting building aesthetics appealed to Phil Mark, director of preservation at Stratford Hall, the historic home of Robert E. Lee. In 2010, the Stratford, Va., plantation installed window film with a VT of 0.69 and a SHGC of 0.50 on 13 single-pane windows in an effort to protect interior finishes and the estate’s collections from UV damage. “You can hardly see it unless you know it’s there,” Mark says.

He anticipates that the film, which took just two days to install, will help reduce the energy used to maintain the buildings’ strict temperature and humidity requirements, which are needed to protect the sensitive nature of the historic contents.

Stress and Shock

Applied films may not always be an appropriate solution. When applied to the room-side face of windows, as is common, films may increase the risk of thermal breakage in glass. Unlike coatings, applied films may not only reflect but also absorb solar and thermal energy, says Mark Dannettel, vice-president in the Los Angeles office of ThorntonTomasetti. While glazing edges hidden by the frame remain cool, he says, films can “artificially heat up the glass in the center of the window.” In insulated glass units, the inboard pane may reflect solar heat back into the unit’s air cavity. “It becomes harder for the outer pane to get rid of the heat gain,” Dannettel says. As a result, the glass can experience excessive thermal stress, which can lead to breakage.

While heat-strengthened and tempered glass are more resistant to thermal stress, films should not be used without consulting a credible film manufacturer or façade expert. “A reputable manufacturer will warrant your base glass for thermal stress failure,” Pettit says. “An authorized window-film dealer will look at a number of factors to determine thermal stress risk, including the type of glass you have, the size of the glass, how the glass was manufactured, frame type, and how the glass was mounted.”

IWFA’s Smith recommends that designers request up front in writing a film manufacturer’s film-to-glass guidelines, and any warranties on its films or on the building, window glass, and seals. Finally, Smith adds, “Ask for an 8-by-10-inch sample of the film that will be installed on your building” to make sure you get what you expect.

Room for Improvement

Applied films continue to advance. In general, Pettit says, “Expect more investment in technologies that are going to help us save energy.” Exterior-applied films that reflect solar heat before window glazing can absorb it are already on the market. Later this year, 3M will also release a film that will mimic the capabilities of a light shelf and redirect natural light further into a building core.

When proper research and measures are employed, Smith says, applied films are an economic and efficient way to improve window performance. “Not only are you using the balance of the window’s physical life,” he says, “but now you’ve also given it an energy life.”

ECO-STRUCTURE's exploration of window technologies continues with “Double Duty,” an article examining the technology of electrochromic coatings. To access it, click here.

Note: This article has been updated since first publication to correct the spelling of Billy Pettit's first name.

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