Understanding
Low-e Window Coatings
You can tell a lot about windows just by looking at them. But if you rely
on your eyes alone, you'll miss an invisible but important energy-saving
feature, the low-e coating.
Low-e coatings, which
are microscopically thin layers of metallic oxide that's bonded to the
surface of a window's glass, are so thin you can see right through them
yet they prevent heat and ultra-violet (UV) rays from passing through
glass. The result? A window with low-e glass does a better job keeping
heat in during the winter and out during the summer.
Letting Light
In
When you look outside on a sunny day, you're seeing the visible part of
the light spectrum. But only 38 percent of the solar energy that reaches
the surface of the earth is visible. The rest of the light that lands
in your yard is invisible. 59 percent is infrared and 3 percent is ultraviolet
(UV rays). Even though they're invisible, infrared and UV rays do affect
the interior of your home. Infrared rays can pass in out out through glass
windows, which increases your home's heat load in the winter and cooling
load in the summer. As for UV light, the same rays that burn your skin
can enter through glazing to fade and degrade your fabrics and furnishings.
Coating Types
A low-e coating works like an invisible mirror to reflect selected portions
of the light spectrum back out or back in through windows. There are three
major types of coatings on the market: soft, hard, and Heat Mirror. Soft
coat is applied to the surface of a finished piece of glass. It's not
durable enough to be exposed to the elements, so it's only used on the
inner surfaces of windows. Hard coat is not quite as energy efficient
as soft coat, but is tough enough to be used on exposed surfaces. Hard
coat is produced by fusing metallic oxide to the hot surface of glass
during manufacture and is found primarily on storm windows and removable
energy panels. Heat Mirror is a proprietary product that's applied to
a thin polyester sheet suspended between the two panes of dual pane window.
The coating reflects radiant heat while the sheet decreases heat loss
by splitting the air space in two.
Energy Ratings
To find out if the windows you're considering are appropriate for the
climate you live in, look at the Energy Star or National Fenestration
Rating Council (NFRC) label. The NFRC label indicates the U-value and
solar heat gain coefficient (SHGC) for that window. The Energy Star label
shows how these ratings compare to the minimum energy-efficiency standards
of the U.S. Department of Energy (DOE). Energy Star breaks the country
into 3 zones. To meet the standards, windows used in areas like Florida
and Southern Texas must have a SHGC rating of .40 or lower and a U-factor
of .75 or lower. In states like Maine and Montana, the DOE says windows
should have a U-factor of .35 or less but has no requirement concerning
solar heat gain. That's because in the far North heat loss is a big issue
but solar heat gain isn't.
The NFRC label also contains information
on visible light transmission (VLT), which can be reduced by some coatings.
The VLT rating indicates the percent of visible light able to pass through
the glazing. Since the whole idea of using glass windows is to admit light,
look for a VLT rating of .60 to .80 (60 to 80 percent). On the plus side,
low-e coatings do block some of the UV light that can fade and damage
furniture. These ratings don't show up on the NFRC label, but can often
be obtained from the window manufacturer.
Cold Climates
Low-e coatings were introduced in the 1980's, and were devised to save
energy in cold climates. Early low-e windows were designed to let light
and solar heat pass through the glazing into the building, and to reflect
heat back into the building when it tried to escape through the glass.
By lowering the U-value or heat loss rating of the glazing, a low-e coating
reduces the amount of energy needed to heat the building. Sitting next
to a window when it's cold becomes much more comfortable because the heat
that radiates from your body is reflected back into the house. What's
more, the glass stays warmer, so moisture is a lot less likely to condense
on the inside of the window.
Hot Climates
The problem with windows designed for cold climates is that they keep
heat in but they don't keep it out. As a result, early low-e windows weren't
much help in the South, where it's harder to stay cool than it is to stay
warm. In the 1990's, manufacturers introduced windows with spectrally
selective coatings. Designed for use in hot climates, windows with these
coatings admit up to 40 percent less solar heat than ones with clear glass
panes. As a result, less heat enters through the glass, so the house is
easier to cool.
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Quick
Tips |
Mixing-and-matching
window coatings
When examing a window's label, keep in mind that there are always
tradeoffs. Increasing energy efficiency by choosing windows with
an extra low Solar Heat Gain Coefficient rating may reduce the
amount of visible light that passes through the glass. However,
while you can see through low-e coatings, they're not completely
invisible. Sometimes they appear as a slight tint on the glass.
To account for different sun exposure, sophisticated designers
sometimes specify different coatings for different windows in
the same house. This can increase efficiency, but you should be
aware that windows with different coatings may appear to have
slightly different colored glass.
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