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Your whole-building design approach will determine what type of
fenestration products—windows, doors, and skylights—you
should use. 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 you identify fenestration characteristics and select products,
the National Fenestration Rating Council (NFRC) has developed a
rating system based on whole product 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:
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 window 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
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% 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 doesn't
rate window 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 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 and homes, see Solar
Heat Gain Coefficient FAQs on DOE's Building Energy Codes Web
site.
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.
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.
For more information on reducing air infiltration, see DOE's Reference
Brief on Replacing and
Weatherizing Exterior and Storm Doors.
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