Windows

Windows, also referred to as glazing or fenestration, affect both how much daylight comes into a building and the building’s heating and cooling loads. The windows you choose, whether in a new build or a retrofit, can make a significant difference in your energy consumption—if properly employed, they can cut annual energy costs by up to 20% in perimeter zones.

In most large commercial buildings, cooling represents the major energy load, so you’ll want glazing that minimizes solar heat gain but doesn’t inhibit daylight. In buildings where heating is the major energy load, you’ll need to choose glazing that minimizes heat loss.

What are the options?
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Understanding the performance parameters will help you choose the right windows for your building.

Window performance metrics

Solar heat gain coefficient (SHGC). SHGC indicates how well a window controls solar radiation. It’s defined as the fraction of incident solar energy that is transmitted through the window assembly as heat gain—how much heat from sunlight is transferred into the interior space.

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. This metric includes how much heat gain can be reduced by shading the window frame as well as the ratio of frame to glass area. Windows with low SHGC values improve comfort for building occupants, 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, an older term still found in some product literature; shading coefficient is defined as the ratio of total solar transmittance to the transmittance through one-eighth-inch clear glass. SHGC is approximately equal to the shading coefficient rating x 0.87.

Visible transmittance. Visible transmittance is the percentage of visible light that makes it through a window. Visible transmittance values range from 0.74 for clear single-pane glass to 0.04 for the most light-blocking glass. Reductions in heat gain (known as SHGC) must always be considered in conjunction with the corresponding reduction in light. For instance, reflective glass products used in many cooling-load-dominated regions create such dim interiors that the space requires almost as much electric lighting as it would if the walls were opaque. That level of electric lighting can release more waste heat year-round than the sun would deliver through normal windows. In this case, to deal with the waste heat from overhead lights, you would have to install more-powerful air conditioners, increasing energy costs.

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. 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. These higher temperatures provide several benefits: moisture condensation is reduced or eliminated, occupant comfort is increased, thermostat setpoints can often be lowered, and the building’s heating system could 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.

Energy moves through windows in four ways: infiltration, conduction, convection, and radiation.

  • Infiltration. Air leaks around the frame, around the sash, and through gaps in movable window parts. Infiltration is foiled by careful design and installation (especially for operable windows), weather stripping, and caulking.
  • Conduction. Adjacent molecules of gases or solids pass thermal energy between them. Conduction is minimized by adding layers of glass to trap air between and putting low-conductivity gases in those spaces. Frame conduction is reduced by using materials that don’t conduct well, such as vinyl and fiberglass.
  • Convection. Pockets of high-temperature, low-density gas rise, setting up a circular movement pattern. Convection occurs within multilayer windows and on either side of the window. Careful spacing of gas-filled gaps minimizes both this issue and conduction.
  • Radiation. Energy that passes directly from a warmer surface to a cooler one. Radiation is controlled with films or coatings that limit these emissions.

Condensation resistance. This metric is a measure of how well a window assembly resists the formation of condensation on its interior surfaces. It is expressed as a number between 0 and 100—the higher the number, the better.

Light-to-solar-gain ratio (LSG). This less-commonly used metric is the ratio of visible transmittance to SHGC. A ratio greater than 1 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.

Types of glazing

The type of glazing used on windows in new construction settings can have a significant effect on a building’s solar heat gain or HVAC load. In a retrofit application, window film is 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.

Standard. Standard, single-pane windows transmit about 88% 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, standard windows are a significant source of heat gain and are also often a source of glare. In insulated buildings, they’re one of the largest sources of heat loss during the heating season.

Tinted. 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 yields a modest reduction in SHGC and reduces visible transmittance. The most common colors for tinted glass are neutral gray, bronze, and blue-green. Black-tinted glass is the worst choice for cooling load reduction because it absorbs visible energy. Green- or blue-tinted glass is more selective than other colors for letting light in while keeping heat out.

Reflective. Semitransparent metallic coatings can be applied to the surfaces of clear or tinted glass, and because these coatings reflect rather than absorb infrared energy, they raise shading coefficients. Reflective glass is most often used in hot-climate applications, where a high level of solar control is critical, but it reduces cooling loads at the expense of daylight. This means more electric light is required, the heat from which may offset the savings. Reflective coatings can be used on all single-pane windows, and some coatings are available that must be sealed inside double-glass units.

Spectrally selective. Spectrally selective glazing is a type of low-e coating that 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%. 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 solar heat gain.

Insulated. Glass by itself conducts heat easily, 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 sashes 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 overall U-factors as low as 0.14.

Electrochromic. This glass is an optical switching technology that can vary its light transmittance. When voltage is applied to the window, it changes from clear to a dark tint in three to five minutes. Reversing the voltage restores the window to a clear state. Although they are more expensive than low-e windows, electrochromic windows are good for solar control, and they can eliminate the need for and cost of interior or exterior shades, which can offset some of their cost.

How to make the best choice
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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 (figure 1).

Figure 1: Glazing options
Glazing products offer a range of optical properties. To minimize cooling loads and maintain daylighting, pick products with high values of LSG. To minimize heat loss, pick products with low overall heat-transfer values (U-factor). Unless noted, these values are for windows with aluminum frames; the listed electrochromic ratings do not include frame effects.
Figure 1: Performance of typical glazing options

Use NFRC-certified windows. The National Fenestration Rating Council (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, visible transmittance, 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 condensation resistance rating may also be listed but is not necessary for certification. The NFRC also created a PCP Site-Built Packet certification program for site-built commercial windows that establishes ratings using computer modeling and accredited laboratories.

Estimate the savings potential. Use a computer simulation program such as DOE-2, the US Department of Energy’s building energy-use simulation software, to calculate the potential savings from an energy-efficient window. This type of tool is necessary to get a holistic view—glazing affects both HVAC and lighting loads, and the lighting loads also have an impact on HVAC, making the calculations difficult.

Evaluate architectural changes. A free simulation tool called the Facade Design Tool is available from the Efficient Windows Collaborative; it can evaluate the effect your window and shading options will have on your energy use. Not every city in the US is represented in the tool, but it designates 15 climate zones that cover the entire country, so data for cities that aren’t specifically represented can be extrapolated from those that are.

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-and-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 3.0 it has some limitations. For example, internal energy load defaults for small office buildings are still being implemented. While these defaults are not customizable, they may still be useful. One other limitation is that the packaged single-zone HVAC system type is the only HVAC system type available for use. Future versions will expand in scope based on user feedback.

Consider chromogenic windows. These windows are also called optical-switching or “smart” windows, and one type—electrochromic glazing—is already commercially available. Other technologies still under development will enable windows to alter their visible 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.

Figure 2: Lighting energy versus cooling energy by glazing type
Most glazing choices involve a trade-off between the requirements for air conditioning and those for electric lighting. For instance, clear glass lets in lots of visible light and solar heat, and hence reduces the need for electric lighting, but it also increases the need for cooling relative to reflective glass. Chromogenic glazings offer improved performance in both parameters.
Figure 2: Lighting energy versus cooling energy by glazing type
What’s on the horizon?
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Several new window types are currently under development, including triple-pane, “insulation-filled,” and evacuated windows.

Triple-pane windows. Glass manufacturing has improved due to the advent of computers and smartphones, and it’s now possible to create thin enough glass panels for triple-pane windows. Window manufacturers are developing processes that will fill the space between panes with argon to offer much higher insulation values—potentially comparable to that of walls.

Insulation-filled 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 don’t provide a clear view. Because of this limitation, these may be best suited to skylight applications.

Evacuated windows. Evacuated windows have the air removed from between the panes, creating a vacuum. This reduces heat transfer, lowering the window’s 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.

Who are the manufacturers?
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More complete listings of window manufacturers are available at the U.S. Environmental Protection Agency’s Energy Star web site and the NFRC web site.

Neither this list nor any mention of a specific vendor or product constitutes an endorsement or recommendation by E Source, nor does any content the Business Energy Advisor constitute an endorsement or recommendation, explicit or otherwise, of your service provider’s various technology-related programs.
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