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Whether
replacing windows in an older house or choosing windows for a new
house, your decisions on what type of windows to buy will be among
the most important decisions you will make in terms of energy
savings. Because of the impact windows have on both heat loss and
heat gain, proper selection of products can be confusing. To add to
the complexity, window glazing technology has changed tremendously
in recent years. The best window glazings today insulate almost four
times as well as the best commonly available windows from twenty
years ago.
Window
technology has improved dramatically in recent years, with the net
result of lowering your energy bills. Some of the most important
energy features of windows are explained below.
Multiple layers of glazing.

Until
the 1980s the primary way manufacturers improved the energy
performance of windows was to add additional layers of glazing.
Double glazing insulates almost twice as well as single glazing.
Adding a third or fourth layer of glazing results in further
improvement. Some of these windows use glass only; others use thin
plastic films as the inner glazing layer(s).
Thickness of air space.
With
double-glazed windows the air space between the panes of glass has a
big effect on energy performance. A very thin air space does not
insulate as well as a thicker air space because of the conductivity
through that small space. Many window manufacturers have increased
the thickness of the air space in their double-glazed windows from
¼" to ½" or more. If the air space is too wide, however, convection
loops between the layers of glazing occur. Beyond about 1", you do
not get any further gain in energy performance with thicker air
spaces.
Low-conductivity gas fill.
By
substituting a denser, lower conductivity gas such as argon for the
air in a sealed insulated glass window, heat loss can be reduced
significantly. Most major window manufacturers offer argon-gas fill
as an option in their most popular windows.
Tinted glass coatings.
Tinted
glass and tinted window films have long been used in commercial
buildings to reduce heat gain through windows. Improved, lightly
tinted windows are becoming more common for the residential market
in Texas. These new glazings reduce the solar heat gain without
reducing visibility as much as older tinted glass and films.
Low-e
coatings. 
More
than any other single improvement, the invention and commercial
development of low-emissivity (low-e) coatings in the 1980s
revolutionized window technology. Thin, transparent coatings of
silver or tin oxide permit visible light to pass through, but they
effectively reflect infrared heat radiation back into the room. This
reduces heat loss through the windows in the winter.
Edge
spacers.
The edge
spacer is what holds the panes of glass apart and provides the
airtight seal in an insulated glass window. Traditionally, these
have been hollow aluminum channels, usually filled with desiccant
beads. Aluminum has extremely high conductivity. That didn’t matter
when the glazing did not insulate very well, but as better
performing glazings were developed, proportionately more heat was
lost through the edges. Since about 1990, a number of improved edge
spacers have come onto the market. Some are made of thin-walled
steel and have a thermal break. Others are made of silicone foam or
butyl rubber. Generally, better edge seals are a low-cost option
when ordering windows, and worth considering.
Many
local utilities provide financial incentives for purchasing ENERGY
STAR qualified windows, doors, and skylights. To find out if rebates
or other incentives are available in your area,
visit the ENERGY STAR Rebate Finder.
Federal Tax Credits
The
Energy Policy Act of 2005 allows consumers to receive a federal tax
credit for making energy efficient improvements on their home,
including installing ENERGY STAR qualified windows, doors, and
skylights. |