U.S. Department of Energy - Energy Efficiency and Renewable Energy
Building Technologies Program – Commercial Buildings
Fenestration
The whole-building design approach will determine what type of fenestration products—windows, doors, and skylights—should be used. Basically, you want to select products with characteristics that accommodate your building's climate, which includes insulating, daylighting, heating and cooling, and natural ventilation needs.
To help identify fenestration characteristics and select products, the National Fenestration Rating Council (NFRC) has developed a rating system based on whole product performance. Product comparisons can be made using the NFRC label, which appears on NFRC-certified products. The NFRC label lists the manufacturer, describes the product, and rates each product in two standard sizes (residential and nonresidential) according to one or more of the following energy performance characteristics:
The U-factor (U-value) measures the rate of heat loss or how well a product prevents heat from escaping. It includes the thermal properties of the frame as well as the glazing. The insulating value is indicated by the R-value, which is the inverse of the U-factor. U-factor ratings generally fall between 0.20 and 1.20. The lower the U-factor, the greater a product's resistance to heat flow and the better its insulating value.
To reduce U-factors, some manufacturers apply a low-E (low-emittance) coating to glazing surfaces. These low-E coatings reduce heat loss, improving both heating and cooling performance. Windows can also be assembled to improve thermal performance. Some assembly strategies include using two or more layers of panes or films, low-conductance gas fills between the layers, and thermally improved edge spacers, which are placed between the panes.
The sash and frame of a window represent 10 to 30 percent of a window's total area, depending on the window size and design. The material used to manufacture the frame can thus impact heat loss and related condensation resistance. In colder climates, in non-residential buildings, where aluminum frames are used, thermal breaks should be specified in order to minimize heat transfer and condensation on the frames. In colder climates, with residential buildings, most products use wood, vinyl, or other non-metallic frames.
Some door frames will also conduct heat readily. For solid doors, insulated metal or fiberglass doors are usually the best choice.
Window coverings—such as shades, shutters, and insulating or storm panels—can help reduce heat loss too. The NFRC does not rate window coverings, but some manufacturers provide R-values for their products.
The solar heat gain coefficient (SHGC) measures how well a product blocks heat caused by sunlight. The SHGC is expressed as a number between 0 and 1. The lower the SHGC, the less solar heat it transmits.
To reduce the SHGC, manufacturers can apply a spectrally selective Low-E coating to the glazing. This type of Low-E coating can reduce heat loss in the winter as well as solar gain in the summer. Reflective coatings and tinted glass can also help reduce the SHGC.
In passive solar designs, south-facing windows with high SHGC ratings might be needed to provide a building with heat in the winter. But a properly designed roof overhang is typically used to reduce the solar heat gain from these windows in the summer.
Some window coverings—shades, blinds, mesh screens, and awnings—can also be used to reduce solar heat gain in the summer or as needed.
For more information about SHGC, see Solar Heat Gain Coefficient FAQs on DOE's Building Energy Codes Web site.
Visible transmittance (VT) measures how much light comes through a product. It is an optical property that indicates the amount of visible light transmitted. The NFRC's VT rating even includes the impact of the frame, which does not transmit any light. VT is expressed as a number between 0 and 1. The higher the VT, the more light is transmitted.
Some tinted glass used to reduce solar heat gain can also reduce the amount of visible light transmitted, which is not good for daylighting. A spectrally selective tinted or coated glazing, however, can help reduce the solar gain while providing as much visible light as clear glass.
When air infiltrates through cracks in a fenestration products assembly, heat loss and gain can occur. The air leakage (AL) rating is expressed as the equivalent cubic feet of air passing through a square foot of the product's area (cfm/sq ft). The lower the AL, the less air infiltration.
For heavily trafficked buildings, air infiltration through doors is an important energy consideration. Revolving doors and double-doored entry ways should be considered in this case.
Window coverings are not effective at reducing air infiltration. Traditionally, the best way to eliminate drafts caused by infiltration is to caulk and weather strip windows, and doors as well.
Glazing systems have a huge impact on energy consumption, and glazing modifications often present an excellent opportunity for energy improvements in a building. Appropriate glazing choices vary greatly, depending on the location of the facility, the uses of the building, and (in some cases) even the glazing's placement on the building. In hot climates, the primary strategy is to control heat gain by keeping solar energy from entering the interior space while allowing reasonable visible light transmittance for views and daylighting. Solar screens that intercept solar radiation, or films that prevent infrared and ultraviolet transmission while allowing good visibility, are useful retrofits for hot climates. In colder climates, the focus shifts from keeping solar energy out of the space to reducing heat loss to the outdoors and (in some cases) allowing desirable solar radiation to enter. Windows with two or three glazing layers that utilize low-emissivity coatings will minimize conductive energy transmission. Filling the spaces between the glazing layers with an inert low-conductivity gas, such as argon, will further reduce heat flow. Much heat is also lost through a window's frame. For optimal energy performance, specify a low-conductivity frame material, such as wood or vinyl. If metal frames are used, make sure the frame has thermal breaks. In addition to reducing heat loss, a good window frame will help prevent condensation—even high-performance glazings may result in condensation problems if those glazings are mounted in inappropriate frames or window sashes.
Fenestration can be a source of discomfort when solar gain and glare interfere with work station visibility or increase contrast and visual discomfort for occupants. Daylighting benefits will be negated if glare forces occupants to close blinds and turn on electric lights, for example, to perform visual tasks optimally.
Facility managers should choose appropriate window technology that is cost-effective for the climate conditions. Computer modeling, using a tool such as DOE-2 or Energy-10, will help determine which glazing system is most appropriate for a particular climate. In coastal California, for example, single glazing may be all that can be economically justified, while in both hotter and colder climates, more sophisticated glazings are likely to be much more effective.
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