Lawrence Berkeley National Laboratory

This study was intended to evaluate the performance of an exterior sun control window film provided by 3M, with field verifications and execution by NGS (the selected film was Exterior Prestige 40), in terms of its ability to reduce the amount of solar radiation transmitted through windows, and therefore reduce space cooling loads, as well as positively impact the experience of occupants in the spaces. Window films are an established technology for improving the energy and comfort performance of windows without replacing the glazing, especially when it comes to controlling glare and solar heat gains through windows. While most film installed in US commercial buildings is applied to the interior surface of the windows, exterior-mounted film can provide improved sun control performance. Relative to interior-mounted film, it prevents more solar radiation from being absorbed by the glass layer(s) of the window and re-radiated to the interior space as long-wavelength infrared. This is particularly the case for insulated glazing units (IGUs) and even more so for IGUs with tinted glazing. The film was evaluated at Building 67 of the Denver Federal Center. Built in 1967, it has 14 floors and encompasses 372,000 square feet of office space surrounding the elevator lobby and core area. The existing windows are tinted double-pane with a hard-coat low-e. The demonstration was conducted in two phases. In the first phase, exterior film was installed on the south, west, and east facades of the 12th floor, with the 13th floor serving as baseline for comparison. These areas of the building were chosen because there were frequent complaints about the space being too hot during the cooling season, with blinds being lowered by the occupants in order to reduce solar heat gain through the windows. Following positive feedback from building occupants and the building management team regarding the film on the 12th floor, film was then installed on the south, west, and east facades of the remaining floors. This represented the second phase of the demonstration. Quantitative performance objectives of the evaluation are shown in Table ES-1, along with the results. Measurements showed a substantial reduction in transmittance solar radiation (averaging 74% throughout a clear sky day in August), showing a clear improvement over the existing windows. An analysis of energy use and peak demand reductions using Energy Plus simulations was conducted for 11 ASHRAE climate zones. Energy use reductions ranged between -2.7% (Duluth, MN) to 7.5% (Phoenix, AZ) in the simulation results, depending on region. Peak energy demand reductions ranged from 6.5% (Houston, TX) to 9.4% (Los Angeles, CA) (electricity), and from -29.1% (Phoenix, AZ) to 3.9% (Baltimore, MD) (gas). Calculated based on annual energy simulations for a medium office building in nearby Boulder, Colorado, the annual energy savings yielded a payback period of 19.32 years, with a saving-to-investment ratio of 0.78 (15 yr expected lifespan). Excluding the costs of any needed heavy equipment (lifts, cranes), the retrofit involves an installed cost of $12.9/ft2-window and no additional annual maintenance costs. It should be noted that these analyses, using GSA blended energy costs, can significantly underestimate the cost-effectiveness of the film retrofit, due to the fact that they don’t take into account the impact of reducing electrical peak demand on the demand charge component of the energy costs. A calculation using typical demand charges for large office buildings in Denver yielded a payback period of 10.51 years and a saving-to-investment ratio of 1.43. The cost-effectiveness metrics obtained for the Denver area, which was where the field test was Lawrence Berkeley National Laboratory: Non-Metalized Exterior Sun Control Film Page 5 conducted, should not be taken as indicative of the film performance in more favorable regions, i.e., regions where cooling is the primary energy concern and hence where this type of film is expected to be more cost-effective. Qualitative objectives and results are shown in Table ES-2. Installation was quicker than anticipated and did not require significant deviations from GSA’s standard practices. One potential caveat was noticed for situations where stepping off of the swing stage might be required. Care is to be taken that applicable safety regulations and procedures are observed. No discernible changes were required in the building’s exterior window washing procedure. Evidence from interactions with floor occupants strongly suggests that the film made the space feel cooler during the cooling season without negative impacts during the heating season. Working with an experienced film installer and conducting pre-installation walkthrough and coordination are important for a trouble-free and quick installation. As exterior installation can involve working at substantial height from the ground, adherence to safety norms and procedures is critical. This technology is expected to yield the highest benefits where the existing windows are double tinted, in buildings and regions where cooling is the primary energy concern, and wherever there is a high cost to even short-term disturbances to the operation and occupants of the interior of the building. For comparison purposes, simulation results for the same film when installed on the interior side of a window, are presented in Table ES-1. Energy Plus simulations were conducted across 11 ASHRAE climate zones in order to determine the reductions in energy use and peak demand for a medium office building in Boulder, Colorado. These reductions ranged from -1.2% (Duluth, MN) to 1.7% (Miami, FL), depending on region. Peak energy demand reductions ranged from 0.6% (Seattle, WA) to 2.2% (Boulder, CO) for electricity and from -10.9% (Phoenix, AZ) to 12.5% (Miami, FL) for gas.