Since the early 1960’s energy saving films have been used as a means to reduce building energy costs. Today, energy saving films are increasingly being used worldwide to lower building energy costs by reducing excessive solar heat gain through windows. Customers are quickly learning that these films are a cost effective option to other window alternatives.
An energy saving window film typically consists of a thin (0.025mm, 0.001 inch) polyester film substrate that has a micro-thin, transparent metal coating applied to one side. This metal coating is applied using vacuum-based technologies such as vapor deposition or sputtering. A second layer of polyester film is laminated over the metal coating to protect the metal. A scratch resistant (SR) coating is applied onto the side of this laminated composite that faces the building interior to protect the film during normal window cleaning. An adhesive layer is applied onto the film side that faces the glass and is protected by a removable release liner until just before the film is applied to the glazing system. UV absorbers are added to the polyester film layers, the adhesive layer, or both to protect from UV degradation.
The appearance of film including color, the level of visible light transmission and degree of reflectivity are determined by the metal coating(s) used. Typically, all-metal energy films are silver-reflective, gray, silver-gray, bronze or light green in color. Visible light transmissions (VLT) can vary from very dark (10 percent) to very light (70 percent), and the visible reflectance can vary from the same reflectance as clear glass (8 percent) to highly reflective (60 percent). The ability of a glazing system to reduce solar heat gain is measured by its solar heat gain coefficient (SHGC). As expected from the variety of films available, the SHGC for window films can vary significantly, from 0.17 to 0.71, as measured on 3mm (1/8 inch) clear glass.
Across all climate zones, energy saving window films offer a better return on investment than low-E window replacement due to the combination of solar performance and significantly lower material and installation costs ($3.00-5.00 per square foot for window films versus $40 or more per square foot for new windows). Studies have shown that, over all four U.S. climate zones (which are indicative of most climate zones globally), for each dollar available for window retrofit or replacement, window film provides 6.6 times greater energy cost savings than total window replacement with new low-E windows. As expected, this ratio is best in the Southern climate zone (10.2 times greater savings with film compared to new low-E windows for each dollar spent) and South/Central zone (7.40 times). Window film even provides impressive results for the North/Central (5.5 times) and Northern zones (4.3 times). A total window replacement with solar-control low-E windows does provide good energy savings (average savings of nearly 10 percent of total building energy costs). But, window replacement provides relatively poor simple payback due to high installation costs. Retrofitting existing windows with energy saving window films provides total building energy cost savings averaging 4.8 percent, but range as high as 9.9 percent in the Southern climate zone. It should also be noted that the return on investment for window film is faster than installing low-E windows. With these factors in mind, the application of energy saving window films should be strongly considered in lieu of window replacement if existing windows are functionally sound (no or few failed seals, limited air leakage, and frames in good condition).
Written by Steve DeBusk, Solutia