Are your energy dollars “going out the window"?
When you evaluate the energy performance of your building envelope, you may find windows near the top of your list for potential improvements. Although new, energy-efficient windows are a logical choice for construction, cost can make them prohibitive as replacements in existing structures. However, advances in materials now make desirable U values affordable with window film.
Window film is a thin, transparent sheet that covers the inside (room-facing) surface of the window. It impedes infrared radiation, thereby reducing heat transfer and cooling load. Window film also tends to reduce indoor temperature near the window, which enhances room comfort.
Because it blocks up to 99 percent of UV radiation, window film helps prevent fading of fabrics and furnishings. It reduces glare (if you’ve ever worked at a computer with the sun beating into your room, you know how vexing glare can be). Window film also provides a safety benefit by improving the window’s ability to resist shattering.
While other window coverings (shutters, shades, drapes, etc.) offer some of the benefits of window film, none allows you to achieve the same combination of energy efficiency, comfort, visibility and safety.
Understanding Window Film Performance
When you evaluate window film for your building, you should know how the film will perform in combination with the window. There are two key metrics that describe this. One, the solar heat gain coefficient (SHGC), measures how much solar heat is blocked. It is expressed as a number between 0 and 1 (lower values indicate more heat blocked). Two, visible transmittance (Tvis), indicates how much visible light passes through. It, too, is expressed as a number between 0 and 1, with higher values denoting more light passage. The National Fenestration Rating Council (NFRC) maintains a database that can help you determine these values for different film and window combinations.
Before you select a window film, you will want to assess the difference between standard and spectrally selective types. Generally, standard films cost less and block more visible light. Spectrally selective films can cost twice as much as standard ones, but they allow more visible light in. With more free daylight at your disposal, you can increase your savings with a dimmable lighting system.
Is Window Film Right for Your Building?
The typical payback period for window film is under 5 years, with product warranties from 5 to 10 years and life spans up to 15 years. Not every window is a good candidate, however. Non-strengthened glass with high solar exposure presents the potential for cracking with dark film. Neither are thick or nonstandard glass types (reflective, laminated, textured, etc.) suitable for window film.
To understand your building’s potential to benefit from window film, consult with an experienced energy services provider. He or she can help you assess occupant comfort, the integration of daylight with dimmable systems, and the performance features of different film products. A professional can help you avoid the pitfalls and arrive quickly at the right solution for your building and budget. At FME, for example, we offer a computer simulation that shows how various window films are likely to affect your energy use, taking into account their impact on cooling and lighting, as well as the interaction of these systems.
3M states that one building saved $350,000 in its first year as a result of the company’s window film. While your savings will probably not be as dramatic, you may save more than you thought possible. Window film offers a very real potential for putting money back into your budget, instead of letting it “go out the window.