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Many
of today's high-performance windows are three to four times more
energy-efficient than windows commonly installed 10 years ago.
To help you make the best choices for your home and budget, here's
a list of recommendations from dealers and window experts. (Also
see: How
to Select an Energy Efficient Window)
Custom-made
vs. stock windows
Dealers
recommend custom-made windows because they are made to fit exactly
in existing openings. This means there are no spaces to fill
in and the viewing area is maximized. It typically takes
about three weeks from the time the order is placed until the installation
can be scheduled, but longer during the busy season, which is from
early fall through December.
Low-emittance
(low-E) coatings
A
low-E coating is a microscopically thin metal or metallic oxide
coating that is applied to the glass by the manufacturer. It helps
keep heat and ultraviolet rays from passing through the window.
Virtually invisible, low-E coatings will keep the house warmer in
winter by reducing heat loss and cooler in summer by blocking heat
from the sun. Most manufacturers apply the coating to the
glass after it is made, although some manufacturers have begun applying
the coating to the glass while it is being made. While
most dealers recommend low-E, Consumer Reports magazine (October
2000) says that low-E coating isn't essential in northern climates.
Double-glazed
windows
Double-glazed
windows have a sealed space between two panes of glass to provide
an added layer of insulation. Compared with a single pane, double-glazing
can cut heat loss nearly in half. There are also triple-glazed
windows, which have an even higher insulating value, but also add
weight and cost of the windows.
Gas
filling
The
gas in the gap between the panes affects the window's insulating
value. Plain air works and is standard in some windows, but many
recommend a heavier gas such as argon, which makes windows more
energy efficient because it's better than air at reducing heat transfer
between the inside and the outside of the home. Some manufacturers
offer a package that includes low-E and argon gas for about $50
more per window. More exotic gases such as krypton is an option,
but most dealers say it's not worth the extra cost.
Welded
frames
Most
dealers recommend frames that are welded over those that are mechanically
fastened or screwed together. A welded frame is about $25
more per window. Also look for neat, well-bonded joints at
the corners of the frame.
Non-metal
spacer bars
Aluminum
has been traditionally used to create the separation between the
two panes of glass. Since aluminum conducts heat and cold,
condensation can sometimes develop. New materials are available
that are better insulators and make the overall window more efficient
and result in less condensation.
Air
infiltration or air leakage rate
Choose
a window that has been tested for air infiltration.. The air infiltration
rate is the amount of air that passes through a square foot of window
area under specific environmental conditions. The lower the air
infiltration rate, the lower the amount of air passing through the
window and thus the less heat loss. The Efficient Windows
Collaborative recommends selecting windows that have an air leakage
rating of 0.3 or below. Select windows with even lower values for
windy locations.
U-value
The
U-value is a measure of the amount of heat that flows through the
window. The lower the U-value, the less heat that will be lost through
the window. The National Fenestration Rating Council rates the energy
efficiency of windows. It recommends a U-value of 0.4 or lower.
The window's U-value should be listed on the label attached to it.
You
also can look at a window's R-value or resistance factor. The R-factor,
similar to the rating used in insulation, is the inverse of the
U-factor. The higher the R factor, the greater the amount
of insulation a window provides. The type of glass, thickness,
number of panes, distance between panes, as well as the manner in
which the panes are connected, all affect the R-factor performance
of the window.
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