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Fenestration: Windows Doors and Skylights

Your whole-building design approach will determine what type of fenestration products—windows doors, and skylights—you should use. Basically, you want to select windows with characteristics that accommodate your building's climate, which includes insulating, daylighting, heating and cooling, and natural ventilation needs.

To help you identify fenestration characteristics of windows and select products, the National Fenestration Rating Council (NFRC) has developed a rating system based on whole windows performance. You can compare products 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:

U-Factor for Windows
Solar Heat Gain Coefficient for Windows
Visible Transmittance for Windows
Air Leakage for Windows

If you are selecting windows for your home, the Efficient Windows Collaborative (EWC) Web site, sponsored by DOE and EWC, is a very helpful resource.

The ENERGY STAR^® Windows Web site provides additional guidance. ENERGY STAR-labeled windows are twice as efficient as the average windows produced 10 years ago. The site provides information on ENERGY STAR windows and where to find them, including a store locator tool.

U-Factor

The U-factor (U-value) measures the rate of heat loss or how well for windows prevent heat from escaping. It includes the thermal properties of the windows 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 for all windows.

To reduce U-factors, some manufacturers apply a low-E (low-emittance) coating to glazing surfaces on their windows. These low-E coatings reduce heat loss, improving both heating and cooling performance of windows. Windows can also be assembled to improve thermal performance. Some windows assembly strategies for windows 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 windows represent 10% to 30% of a windows total area, depending on the windows 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.

Windows coverings—such as shades, shutters, and insulating or storm panels—can help reduce heat loss too. The NFRC doesn't rate windows coverings. But some manufacturers provide R-values for their products.

Solar Heat Gain Coefficient

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 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 design, 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 windows covering—shades, blinds, mesh screens, and awnings—can also be used to reduce solar heat gain in the summer or as needed.

Visible Transmittance

Visible transmittance (VT) measures how much light comes through a product. It's an optical property that indicates the amount of visible light transmitted. The NFRC's VT rating even includes the impact of the frame, which doesn't 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.

Air Leakage

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.

Windows 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.

Windows can be a large part of the building envelope.

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