Window Film: A Window of Opportunity
By Rachael Zimmermann - December 2006 - Energy Efficiency
There are many examples of wasted energy in a facility — lights left on in rooms without occupants, or inefficient HVAC systems, for example. One source that often goes unrecognized is the energy required to cool space heated by sunlight coming through windows. Roughly one third of an average building’s cooling load is due to solar heat gain through windows, according to the Building Energy Databook, an annual report of data averages published by the U.S. Department of Energy.
Of course, windows provide views and sunshine that can reduce the need for artificial lighting. Even so, more daylight also means more heat, and this requires cooling to keep occupants comfortable.
One way to reduce solar heat gain and improve the energy efficiency of a building is window film. Solar control window film is applied to the inside of a window where it reflects and absorbs heat. In addition to reducing the need for cooling, window film helps create an even, comfortable temperature throughout a building.
A building’s directional orientation relative to the sun can create “hot spots,” or areas that receive significantly more sunlight than others. Depending on the sophistication of a facility’s HVAC system, this can create challenges for maintaining occupant comfort.
“If a building has mainly large glass areas facing north and south, additional cooling may be needed to maintain comfort for offices on the southern exposure,” says Steve Debusk, energy solutions manager, CP Films. “In situations like this, offices on the northern exposure are typically overcooled and uncomfortable. By reducing the solar heat gain on south facing offices, the air conditioning system is better able to maintain comfortable conditions for all.”
Excess solar heat gain can also be a problem even in colder temperatures, says John Miller, a spokesman for V-Kool.
“South- and west-facing walls can be getting too much heat, even in winter, which can require air conditioning,” says Miller. That can be true even on days when heat is running in other parts of the building.
How Window Film Works
Solar control window film reduces heat gain by blocking solar radiation. Solar radiation, or solar energy, is made up of three components: ultraviolet radiation, visible light and near-infrared radiation. Near-infrared radiation makes up 53 percent of the solar spectrum, visible light 44 percent, and ultraviolet 3 percent. When rays from the sun hit a window, some of the energy is absorbed and some is reflected by the window, but most is transmitted through the glass. According to the International Window Film Association (IWFA), a pane of clear glass reflects about 6 percent of solar radiation, absorbs 5 percent and transmits the remaining. When this energy enters a space and is combined with the ambient heat already present in a room — due to people, computers and reradiated heat absorbed by furniture and carpeting — the excess heat can make an uncomfortable difference, particularly for people near windows.
Installing solar control window film increases the amount of solar energy both reflected and absorbed by the window. Most window films have a thin metallic coating, made up of aluminum, stainless steel, silver or a combination of these, that reflects and absorbs solar radiation. Of the radiation absorbed, most of it is reradiated outward, though some is radiated into the building.
The most important performance measurement of the heat rejection ability of solar control window film is the solar heat gain coefficient (SHGC), says Darrell Smith, executive director of the International Window Film Association. SHGC measures the total amount of solar energy transmitted into the room. This includes both solar energy directly transmitted through the glass and solar energy absorbed and radiated into the building by the film. The heat rejection ability of window film can be as high as 80 percent, says Smith. The amount of energy reflected and absorbed varies depending on the types and quantities of metals used in the film.
It is important to note that the percentage of solar energy rejected and the percentage of near-infrared radiation rejected are different measurements. A measurement of near-infrared rejection only takes into account radiation in the near-infrared range, while solar energy rejection measures both near-infrared and visible light rejection. Visible light, when transmitted through glass and absorbed by furniture or carpet, also contributes to heat gain.
For example, says Smith, if two window films have an equal SHGC, and one rejects more near-infrared radiation than the other, it would also transmit more visible light. This is because more of the solar energy rejected is in the near-infrared range.
The near-infrared and visible light rejection capabilities of window films vary, and the combination that is best will be determined by the goals for the application, and what results are desired.
A reduction in solar heat gain can translate directly into fewer kwh used for cooling. What’s more, most utilities have a separate demand charge based on the highest monthly rate of electricity use for a year, or the peak demand. This charge is in addition to the regular cost of electricity. By reducing the amount of cooling needed during peak periods, demand charges can also be reduced.
“From a building management perspective, you can reduce kwh, make tenants more comfortable and reduce demand charge,” says Jim Mannix, business development manager, 3M.
Because of its ability to help save energy, window film is recognized and encouraged as an energy-efficient retrofit.
“The ability to reduce energy costs for buildings is widely accepted by many utility companies that offer significant incentives and rebates for installation of window films,” says Debusk. “Window films are used extensively by energy service companies that implement energy-efficiency improvements to facilities.”
The Full Spectrum
Although reduction in solar heat gain and cooling costs are the primary benefits of window film, there are other advantages as well. The near-complete rejection of UV rays can decrease fading of furniture and mitigate the harmful effects UV rays have on building occupants.
“All quality window films reject between 95 and 99 percent of ultraviolet energy,” says Smith. “UV is not considered part of heat gain, but it can be destructive as well as cause long-term exposure problems to individuals.”
Window film also reduces the amount of visible light that comes through a window, which can help reduce glare on computer screens and other reflective surfaces. The amount of visible light transmitted is part of the SHGC, so the amount of visible light rejected will depend on how much near-infrared radiation is rejected. Visible light transmission can range from 3 to 5 percent for privacy films, and up to 88 percent for clear UV films, Smith says.
While blinds or shades can also be used to limit visible light, window film can achieve this goal while creating a more uniform look on the outside of the building. It also preserves the view for occupants.
“The building occupant is looking for a view and is paying for a view,” Mannix says. “You want the space to be comfortable while maintaining that view.”
Finding the Right Film
Knowing the types of windows a facility has and the specific energy needs of the building will help to determine the best film. For example, for facilities that want to limit solar heat gain while letting in visible light, spectrally selective films are available that allow more visible light to be transmitted while still rejecting near-infrared radiation. Spectrally selective films have visible light transmission between 35 and 75 percent, says Smith. These types of films can work for buildings originally designed for daylighting.
Knowing the characteristics of a facility’s windows is also important to ensure that the film selected won’t damage the window. Smith says that manufacturers use film-to-glass tables as guides to what type of window film is best for a certain window.
“Film recommendation varies by type of glass, window construction, and age and condition of the window system,” he says.
Carlos DeCespedes, CEO, PWTinters, agrees, noting that thermal stress may cause glass failure if the wrong film — one that absorbs more energy than the window can tolerate — is applied. “You have to have the right combination of film and windows,” he says.
Security and safety window films are also making headway in the window film market. While some security film blocks solar radiation, its primary purpose is to prevent shards of flying glass from injuring occupants if the window breaks. Causes of breakage can include bomb blasts, hurricanes, seismic events, or people attempting forced entry through a window, says Randy Garcia, safety and security films product manager, CP Films.
“A bomb blast causes a building to implode,” says Daniel Leclair, a security consultant with SAKO Associates. “Security window film encapsulates the glass, so the glass doesn’t shatter into small pieces that go flying.”
Shattered glass can be a risk to buildings that may not necessarily be a target for a bomb blast, but are in the vicinity of a building that is.
“The attack on the Murrah building in Oklahoma City proved that a bomb blast blocks away can have devastating effects,” says Wayne Staley, vice president and general manager, Commercial Window Shield. “Two hundred and fifty eight buildings as far as 10 blocks away had major glass damage.”
At 4 to 14 mil, security film is significantly thicker than solar control film. While solar control film is applied only to the part of the window that is visible, security film is installed into the window system itself by a process known as anchoring.
Two types of anchoring are available, known as wet-glazed and mechanical installations. A wet-glazed installation involves removing the rubber around the window from the gasket and replacing it with a structural silicon sealant that fills the space between the window and the frame, says Smith. A mechanical attachment involves overlapping the film around the edges of the window and securing the film with bolts to an internal frame. Both methods are meant to ensure that the film will hold glass fragments together and to prevent flying glass.
The value of security film in protecting building occupants has been recognized by the federal government, especially after the events of 9/11, says Leclair.
“All government buildings have some type of window film or protective glazing on the windows,” he says. “The majority of federal buildings have wet-glazed film application as a requirement.”
DeCespedes says that security film is also a recognized benefit in hurricane-prone areas like Florida because of the round the clock protection it provides.
“The benefit of safety film is that it provides protection twenty four hours a day, seven days a week,” he says.