Windows:
Understanding the Label
The first step to buying the right window is understanding how to read
the label.
Just because windows look the same doesn't
mean they'll give you the same performance. Some windows are much more
energy efficient than others, which will decrease heating and cooling
costs while increasing the comfort of your home. If you live in a cold
city like Boston, switching from low-efficiency to high-efficiency windows
will cut your heating costs by as much as 40 percent. If you live in a
hot climate like Phoenix, better windows can knock up to 30 percent off
your cooling bills.
Read the Label
The best way to determine the energy efficiency of a given window is to
compare rating labels. Nowadays, most windows are rated by organizations
like the National Fenestration Rating Council (NFRC) or the Department
of Energy (DOE). Better quality windows will have NFRC labels. This label
indicates that the manufacturer submitted similar units for laboratory
testing. The tests measure the window's U-factor or conductivity, solar
heat gain coefficient (SHGC), and visible light transmittance (VLT). In
addition, some windows receive the DOE's Energy Star Label This label
uses color-coding to indicate which windows meet minimum performance standards
for energy efficiency in each region of the country. The minimum standards
for the North are a U-factor of .35 or lower with no requirement for SHGC.
Minimum standards for the central region are a U-factor of .40 or lower
and a SHGC of .55 or lower. In the South, a U-factor of .75 or lower and
a SHGC of .40 or lower will earn you a star.
U-Factor
In the winter we expect windows to keep heat in the house, but in the
summer we expect them to keep it out. You can gauge a window's ability
to do this by looking at the U-factor. The U-factor is a measurement of
how quickly heat will pass or conduct through a given window. The lower
the U-factor, the harder it is for heat to pass through the window. For
example, a single-glazed aluminum window readily conducts heat because
it has a relatively high U-factor of about 1.30. A double-glazed wood
window does a better job keeping heat where you want it because it's rated
at around .30.
Solar Heat Gain
Heat passes in and out through your windows every day of the year. It
also conducts as radiant energy, which can be a big problem in the summer
when you want it to be cool inside. The solar heat gain coefficient (SHGC)
is a measure of how much solar heat can pass through a window. If a unit
has a rating of .50, it means that the window admits 50 percent of the
heat that strikes it. This measurement covers the whole window, so the
amount of heat that gets through depends on the type of glass and the
width of the frame parts. A typical value for a double-glazed window would
be between .30 and .60. The .30 window would admit half as much solar
heat as the .60 window.
Visible Light Transmittance
Strange as it sounds, only a certain portion of visible light can pass
through a pane of glass. The more that gets through, the brighter the
inside of your house. The percentage of available light that passes through
the window is measured as visible light transmittance (VLT). For example,
a window with a VLT number of .50 only admits 50 percent of the available
light. Again, ratings are for the entire window, so remember that wide
frames block more light than narrow ones. The glass coatings used to prevent
solar heat gain or radiant heat loss also reduce the amount of light that
can get through the glass. The VLT will help you figure how much light
you're giving up when you opt for higher energy efficiency.
Low-e Coatings
A low-e coating is an incredibly thin layer of metal applied to the surface
of the glass. The coating is so thin that you can see through it, yet
it has the ability to reflect heat rather than conduct it. In a cold climate
these coatings help keep the house warm by preventing the escape of radiant
heat. In a hot climate their role is to keep heat from entering by reflecting
it back out through the windows. A wide variety of low-e coatings are
available. Spectrally selective coatings are especially good for cooling
climates where they reduce solar heat gain without blocking an excessive
amount of light.
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Quick
Tips |
Leakage
or Infiltration
If you live in an older home, you probably know all about leaky
windows. They admit drafts, and drive up heating and cooling costs.
While it's a given that old windows leak, it's hard determine how
tight a new window might be. That's because most windows aren't
rated for. If you live a high wind or wind-driven rain area such
as the coast, you can check the window for minimum tightness standards
set by the Window and Door Manufacturers Association (WDMA) or the
American Architectural Manufacturers Association (AAMA). Both groups
set standards for leakage with regard to air and water, but beyond
the coastal areas few people really need to worry about it. |
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