Energy Advisor
Your guide to energy products
for commercial buildings
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Energy Advisor Your guide to energy products |
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 fact, the energy lost through windows in U.S. buildings represents a flow of energy greater than that moving through the Alaskan oil pipeline. Energy efficient windows used in either new or retrofit situations can cut annual energy costs by up to 15 percent 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 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. (See sidebar for definitions of glazing performance parameters—shading coefficient, solar heat gain coefficient, visible transmittance, luminous efficacy constant, and R-value.)
Standard glazing. Standard, single-pane windows transmit about 88 percent of the solar light that strikes them and offer a resistance-to-heat R-value of less than 1. 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 yields only a modest shading coefficient—in the range of 0.5 to 0.8. The most common colors for tinted glass—bronze and gray—block lifght and solar near-infrared heat in roughly equal proportions. 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 for single-pane applications, while some coatings must be sealed inside double-glass units.
Spectrally selective glazings. Spectrally selective glazings are a variation on earlier low-emissivity ("low-e") glazing 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. Values of Ke greater than 1.0 indicate that a glazing is spectrally selective. Typical Ke values for these "second-generation," selective low-e coatings on clear glass range from 1.1 to 1.3, with daylight transmittances as high as 65 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. Advanced products for high daylight transmittance with high solar heat rejection have Ke, on clear glass, greater than 1.4 and can approach Ke values of 1.7 in conjunction with green-tinted or other specially colored glass.
Retrofittable window films. 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 are available by applying after-market films.
Insulated glazing. Glass by itself has high heat conductivity, but by trapping air or an inert gas such as argon or krypton between layers of glass, manufacturers can produce glazing with double the ordinary resistance to heat conductance (Figure 1). With insulated windows, the thermal weak point becomes the edge of the unit and 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.
Energy moves through windows in four ways: air leaks, heat convection, radiation, and heat conduction.
Source: Platts
Establish which loads 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 daylighting 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.
Get data from 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. NFRC-certified window ratings appear in the NFRC Certified Products Directory and on a label on the window itself.
Estimate the savings potential. A computer simulation 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.
Research is underway at a number of institutions to develop "smart windows," also known as chromogenic or optical switching windows. The technology will enable windows to alter their transmittance in response to temperature (thermochromic) or light (photochromic) fluctuations or in response to small electrical currents (electrochromic). Figure 2 shows the potential performance of these emerging technologies.
Most glazing choices involve a trade-off between the requirements for air conditioning and 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 increases the need for cooling relative to reflective glass. Chromogenic glazings have the potential to improve performance in both parameters.
Note: a. Lighting savings assume the use of a switched or dimmable electric lighting control.
Several types of glazing materials are also under development that use optical ingenuity to reject or redirect incident solar radiation for better daylighting control. A typical application is shown in Figure 3.
Using refractive and reflective optics allows direct daylight to be distributed deeper into a building's core.
Source: Platts
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