Solar heat gain and glare are among the few downsides of daylighting, negatively impacting occupant comfort levels and driving up cooling-related energy consumption. Most low-E window films combat this problem by incorporating metallic coatings or dyes that transmit less solar energy through the window, which reduces glare and lowers heat gain.
But none have impacted the window’s insulating value or thermal conductivity – until now.
The GSA recently tested a new applied window film with a room-facing low-E surface added to the traditional technology. It slashed an average of 29% from perimeter HVAC use, which could make a sizable dent in the estimated 34% of commercial building HVAC energy lost to windows.
How Window Film Works
The new applied film technology used in the field test provides both solar gain control and thermal insulation thanks to an additional low-E surface that faces into the room. The applied film has a very low emittance rating of about 0.05, significantly lower than the emittance of typical glass (0.84) and of some of the low-E applied films (which are typically not lower than 0.4). This allows it to achieve a U-factor of 0.58 BTU/hour-square foot-degree F., nearly half the thermal conductivity of a typical single pane window at 1.025 BTU.
“The applied film, with its low-E surface facing the room, helps lower the solar heat gain coefficient as a greater fraction of the energy absorbed at the window is rejected to the outside,” the GSA explains in the field study. “Occupant thermal comfort is another benefit, as the radiation exchange is diminished between the occupants and the relatively higher or lower surface temperatures of the window for both heating and cooling conditions.”
The film was tested at two existing GSA buildings – the Hansen Federal Building in Ogden, UT, and the Cabell Federal Building in Dallas. The two radically different climates covered cold winter, hot summer and mixed seasonal conditions so the GSA could determine how they performed in every climate. Both buildings had a window-to-wall ratio between 30-40%, and windows with original glass conditions on both structures were monitored along with windows retrofitted with film as a control group.
The manufacturer offered two variations of the applied film with different optical properties (namely the amount of visible spectrum transmission), but for the purposes of direct comparison, the research team used the version with the lowest solar gain on both buildings. It features a solar transmission rating of 0.21 and a visual spectrum transmission rating of 0.34. Products with higher gain are ideal for applications where passive solar gain is more important than solar control, as well as north-facing buildings where high visible transmission is important.
The GSA team also distributed occupant surveys to office workers in window retrofit areas before and after the installation to compare comfort levels. The study included field monitoring during summer and winter, plus annual energy modeling.
The Results: Window Film Works
The room-facing low-E surface “presents a compelling low-cost retrofit to add thermal insulation performance to windows in addition to the solar control of typical applied film products,” the GSA notes. The agency found a payback of 2-4 years when the film is applied to clear windows with no existing film, and the steeper savings help make the case for choosing the higher-performance film over a less expensive, but lower-performing version, especially on windows that need either new film or a replacement for existing film at the end of its service life.
The film also makes an effective retrofit on lower-performing double glazing that doesn’t have a low-E coating between the panes in addition to performing well on single glazing base windows, the GSA says. Double glazed windows with a film retrofit return lower energy savings, but the payback time is still favorable.
“The unique insulation performance enhancement offered by the low-E applied film allows it to be less sensitive to summer/winter solar heat gain tradeoffs and thus fit a wider deployment scope than other solar control films,” the GSA says. “For heating and cooling combined, savings of about 20% over a standard tint film and up to 30% relative to single clear glazing were consistently demonstrated across the widely varied climates studied. Heating savings (around 10%) were smaller than cooling savings (30-40%).”
Above image: Infrared Thermograms show the impact of low-E film tested on the south facade of the Hansen Federal Building in Ogden, UT, in the summer. The windows on the left are untreated, while the ones on the right are retrofitted with film. The temperature rise of the patches on the floor shows the difference in solar transmission. Photograph by GSA