Liquid-Applied Absorbing Solar Control Window Film Retrofit

Nationwide, on an annual basis, windows in commercial buildings are responsible for 0.96 quadrillion BTUs (quads) of heating energy and 0.52 quads of cooling energy (Apte 2006). This is equal to about 1.5% of the total energy consumption by the United States in 2011, and is equivalent to the energy consumed by more than 8 million U.S. households (US EIA 2012). There is substantial potential for reducing both the heating and cooling energy use in existing commercial buildings associated with windows by using a wide range of technologies and strategies. This study focuses on solar control window retrofit technology that primarily targets the energy savings potential of reducing the cooling load. A previous study estimated that there is a potential to save 0.32 quads (or 62%) of the window-related cooling energy if the entire U.S. commercial building stock were to be retrofitted with typical solar control, low-e double pane glass units (Apte 2006). Replacing the entire window captures the greatest savings, but can be very expensive. Retrofit applied films do not usually change the insulating value of a window; however, they directly reduce the solar gain through the window. A significant portion of potential cooling energy savings is available through reducing the solar heat gain properties of a window, which can be accomplished with a solar control film retrofit, applied directly to existing glass. Retrofit films have a much less complicated, and less expensive, installation than a complete window replacement. It is important to recognize that reducing solar gain through the window, while it saves cooling energy during portions of the year, can contribute to increased heating energy consumption in winter, such that the annual energy performance of the retrofit for the particular building and climate must be considered. Solar control films are often applied to improve thermal comfort of occupants near the window and to reduce glare from high transmission glazing. A larger potential for energy savings is expected for a solar control film retrofit when the building is in a warmer climate with mild winters and the building is cooling load dominated as a result of internal loads, window area, orientation, solar exposure, and other building and user specific factors.