Windows, more formally referred to as glazing or fenestration, affect the daylight that can be effectively used in a building as well as heating and cooling loads. The windows you choose can make a significant difference in your energy consumption—in both new or retrofit situations, they can cut annual energy costs by up to 20 percent in perimeter zones if properly employed.
Cooling represents the major energy load in most large commercial buildings, so glazing that transmits adequate light for daylight activity while minimizing solar heat transmission is usually best. In buildings where heating is the major energy load, glazing should be carefully chosen to minimize heat loss and, in some cases, configured to increase passive solar heat gain while maximizing daylighting.
There are a host of options for energy-efficient windows. Initially, it might be confusing, but once buyers or specifiers get a handle on the various performance parameters used in assessing glazing, they can better identify their individual needs.
There are several standard metrics currently used to evaluate window performance.
Solar heat gain coefficient (SHGC). The SHGC indicates how well a window controls solar radiation and is defined as the fraction of incident solar energy that is transmitted through the window assembly as heat gain. A low SHGC indicates low heat gain; values range from 0.72 for single-pane windows with aluminum frames to 0.09 for the best-performing windows. It includes the effects of shading from the frame and the ratio of frame to glass area. Windows with low SHGC values improve comfort for building occupants near sunlit windows, lower the total cooling load of the building, and help smooth out the difference in cooling loads between perimeter and core zones. This metric replaces the shading coefficient (SC), an older term still found in some product literature and defined as the ratio of total solar transmittance to the transmittance through one-eighth-inch clear glass. The SC times 0.87 approximately equals the SHGC.
Visible transmittance (Tvis). Tvis is the percentage of visible light that makes it through a window. Reductions in SHGC must always be considered in conjunction with the corresponding reduction in Tvis. For instance, reflective glass products used in many cooling-load-dominated regions create such dim interiors that they need almost as much electric lighting as if the walls were opaque, and that level of electric lighting can release more waste heat year-round than the sun would deliver through normal windows. Air conditioners must then be sized to remove heat generated by lights that wouldn't be needed during the day if the right windows were used. Tvis values range from 0.74 for clear single-pane glass to 0.04 for the most light-blocking glass.
Heat transfer (U-factor). U-factor, sometimes called U-value, indicates the total rate of heat flow from conduction, convection, and radiation through a window due to the difference between indoor and outdoor temperatures (Figure 1). Windows with low U-factors have low heat-flow rates, which lead to significantly higher radiant temperatures on the window's inner surfaces in cold weather. This provides several benefits: moisture condensation is reduced or eliminated, occupant comfort is increased, thermostat setpoints can potentially be lowered, and the building's heating system may be downsized. Because the U-factor varies when measured at different points of the glass—the center, the edge, or the frame—the "overall U-factor" is often used to give the insulating value for the entire window assembly. Overall U-factor values range from approximately 1.25 for clear single-pane windows with aluminum frames to 0.14 for the highest-performing windows.
Condensation resistance (CR). Expressed as a number between 0 and 100, the CR measures how well a window assembly resists the formation of condensation on its interior surfaces.
Light-to-solar-gain ratio (LSG). This less-commonly used metric is the ratio of Tvis to SHGC. A ratio greater than one indicates that the window transmits more light than heat. Windows with low LSGs are more often used in passive solar heating applications, whereas windows with high LSGs are used to help prevent heat gain.
Standard glazing. Standard, single-pane windows transmit about 88 percent of the sunlight that strikes them and have the highest heat transfer rates: a center-of-glass U-factor of approximately 1.09 and an overall U-factor of approximately 1.25, when used with standard aluminum frames. In the cooling season, they are a significant source of heat gain and are also often a source of glare. In insulated buildings, they are one of the largest sources of heat loss during the heating season.
Tinted glazing. Tinted windows, also known as heat-absorbing glass, block heat transmission through bulk absorption in the glass itself. Unfortunately, this also causes the glass temperature to rise, increasing the radiation coming off the window into the conditioned space. The result is that tinting by itself only yields a modest reduction in the SHGC while also reducing visible transmittance by a slightly larger amount. The most common colors for tinted glass are neutral grey, bronze, and blue-green. Black-tinted glass is the worst choice for cooling load reduction, because it absorbs much more visible energy than near-infrared. Green or blue-tinted glass is more selective than other colors for letting light in while keeping heat out.
Reflective glazings. Semitransparent metallic coatings can be applied to the surfaces of clear or tinted glass. They have better shading coefficients because they reflect rather than absorb infrared energy. However, most reflective glazings block daylight more than solar heat. Reflective glass has achieved its greatest market penetration in hot-climate applications, where a high level of solar control is critical. However, reflective glass reduces cooling loads at the expense of daylight transmittance, so the reduction is offset somewhat by the heat created by the additional electric lighting required. Reflective coatings are available both for single-pane applications and in some coatings that must be sealed inside double-glass units.
Spectrally selective glazing. Spectrally selective glazing is a variation on earlier low-emissivity (low-e) window coatings, which were designed to improve the insulation performance of windows while maximizing solar heat gain. Now the selective coatings can maximize or minimize solar gain, or achieve a balance anywhere in between. Typical LSG values for these second-generation selective low-e coatings on clear glass range from 1.2 to 2.0, and one glass product from PPG Industries, Solarban 70XL, has an LSG of 2.3 with a daylight transmittance of 63 percent. These coatings can be combined with tinted glazings, offering an extensive range of aesthetic options, all with state-of-the-art performance in transmitting daylight while minimizing invisible solar heat gain.
Retrofittable window film. In a retrofit application, window films are a proven low-cost method for reducing cooling load with relatively low risk. Many of the benefits of solar-control glazing can be gained by applying after-market films to standard windows.
Insulated glazing. Glass by itself has high heat conductivity. But by trapping air between two clear panes, manufacturers can produce glazing with half the heat flow of standard glazing—a U-factor of about 0.6. Trapping an inert gas such as argon or krypton between two or more layers of glass can further reduce the U-factor. With insulated windows, the thermal weak point becomes the edge of the unit where the glass meets the window frame. To improve performance, manufacturers use thermal breaks in metal frames, increase the use of wood and clad-wood sash and frames, and increase the use of frame materials with lower thermal conductivity, such as vinyl. By combining inert gases with multiple panes, low-conductivity frames, and low-e coatings, manufacturers have achieved U-factors as low as 0.14.
Electrochromic glazing. Commercially available for the first time in 2006, electrochromic glass is an optical switching technology that can vary its transmittance. When voltage is applied to the window, it changes from clear to a dark tint in 3 to 5 minutes. Reversing the voltage restores the window to a clear state. Although they are more expensive than low-e windows, electrochromic windows are used both for fenestration and for solar control. They can eliminate the need for and cost of interior or exterior shading devices, somewhat offsetting their higher cost.
Determine the loads that dominate. When cooling loads have the dominant impact on energy use, which is the case for most large commercial buildings, then the best products are those that maximize daylight while keeping summer heat out. When heating loads dominate, then the insulating value of the window is most important. Table 1 presents typical values for different window options.
Use windows certified by the National Fenestration Rating Council (NFRC). The NFRC is a coalition of industry and public-sector groups that works to standardize and improve the performance ratings of all fenestration products, including windows, doors, and skylights. The NFRC certifies the SHGC, Tvis, and U-factor ratings for prebuilt window assemblies, which are then listed in the NFRC Certified Products Directory and on a label on the window itself. A CR rating may also be listed, but is not necessary for certification. The NFRC also has a certification program for site-built commercial windows that utilizes computer modeling and accredited laboratories to establish ratings.
Estimate the savings potential. A computer simulation program such as DOE-2, the U.S. Department of Energy's building energy use simulation software, will usually be necessary to calculate the potential energy savings from an energy-efficient window. This is because the glazing affects both HVAC and lighting loads, and the lighting loads also have an impact on HVAC.
Evaluate architectural changes. An easy-to-use free simulation tool based on DOE-2 is available from the Center for Sustainable Building Research at the University of Minnesota and Lawrence Berkeley National Laboratory. Called the Facade Design Tool, it allows a user to evaluate the impact various window and shading options will have on energy use. However, it only provides simulation results for six representative U.S. cities.
Model residential applications. Software developed by the Lawrence Berkeley National Laboratory (LBNL) for residential applications, RESFEN, estimates savings given the net performance metrics of a window/film combination as well as other variables such as house type, geographic location, and energy costs.
Model commercial applications. Similar to RESFEN, the COMFEN tool from LBNL is for commercial applications. Although helpful for some applications, at version 1.0—the first to be released—it has a few shortcomings such as its limited library of glazing system configurations and geographic locations. Future versions will expand in scope based on user feedback.
Research is under way at a number of institutions to develop "smart windows," also known as chromogenic or optical-switching windows. One of these technologies, electrochromic glazing, is already commercially available. Other technologies still under development will enable windows to alter their transmittance in response to temperature (thermochromic) or light (photochromic) fluctuations. Figure 2 shows the potential performance of these emerging technologies as well as that of electrochromic glazing.
Also under development are insulation-filled and evacuated windows. Insulation-filled windows use translucent fillers, including aerogels—a silica-based material with the highest known insulation value of any solid. These fillers retard heat transfer through a window, but do not provide a clear view. Thus they may find application more in skylights than in windows.
Evacuated windows have the air removed from between the panes, creating a vacuum. This reduces heat transfer, lowering the U-factor. However, a vacuum creates structural pressures on a window that, in combination with normal pressure variations caused by wind and vibration, can compromise the window's integrity. A possible solution to this problem is the use of small glass pillars between the panes, which provides some stability but also reduces clarity.