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Windows provide several functions for your home: daylighting,
solar heating,
and ventilation. They also serve as part of the building envelope,
providing a barrier between your house and the outdoors. You should
take all these functions into consideration when choosing a replacement
window. Also look for the ENERGY
STAR label.
The Efficient Windows
web site is an excellent source of information to help you choose
energy-efficient windows. In terms of the building envelope, the
window's U-factor (a measure of its insulation ability) is critical.
Look for National Fenestration Rating Council (NFRC) rated U-factors.
NFRC ratings represent the whole window's performance (not just
the glass area) are thus more accurate and representative of real
conditions.
Glazings in both windows and glass doors can have dramatic effects
on the energy performance of a house. A wide variety of coatings
and configurations are now available to achieve a range of goals.
In addition to a glazing's U-value, the other important factors
are the daylight transmittance (the amount of visible light the
window lets in) and the solar heat gain coefficient (the amount
of heat the glazing lets in—glazings with low solar heat gain
coefficients allow less heat in).
Glazings are now available with high daylight transmittances and
low solar heat gain coefficients, allowing daylighting without heat
gain. These windows are a good choice when solar heating is not
the goal. For windows intended for solar heating, obviously a high
solar heat gain coefficient is needed.
"Low-e" (low-emittance) coatings are a recent innovation for glazings.
Low-e coatings reduce the heat transmitted through the pane, which
significantly lowers the U-value (in other words, increases the
insulating ability). There are two types of low-e glass, high solar
transmitting (for climates where utility bills are highest in winter)
and low-solar transmitting (for climates where utility bills are
highest in summer). Choosing the right type of low-e coating for
your home is critical. You may need different coatings for the various
windows in your home depending on their location. Work with a reputable
contractor or retailer to choose the appropriate glazings for your
climate and requirements.
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