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| Thermogram of a Passivhaus building, with traditional building in background. (Photo credit: Wikipedia) |
You’re ready to replace your windows – you’ve determined your needs and your budget – but when you start shopping, you find yourself confused by terms like “Low-e”. You’re not alone! While you may understand that it’s important to look for windows with a Low-e factor, you may be wondering what that means and why it’s important. No explanation of Low-e (short for Low-emissivity) would be complete without the definition of a few other key terms, such as “thermal radiation”, “reflectivity”, and “opacity”.
Emissivity
The “E” is short for emissivity, specifically thermal
emissivity. Low-emissivity windows and
doors cause heat to be reflected off, rather than allowing it to collect and
“seep through” (be emitted) into the home. Perhaps an easier way to think of it
is to consider a surface that has a high emissivity factor: asphalt. If
you’ve ever stepped into a parking lot in Florida in August, you’ve felt the effects
of high emissivity. The asphalt absorbs and emits heat at a ratio .90 to .10.
In other words, 90% of the heat directed at the asphalt is absorbed and emitted,
and only 10% is reflected. You certainly wouldn’t want the glass in your home’s
windows and doors to have that level of emissivity! But plain, uncoated glass
has an even higher level of emissivity than asphalt: .93. This is why Low-e is
such an important factor in choosing your new windows and doors. But how does
it work?
Thermal Radiation
Everything with a temperature greater than absolute zero (−273.15° Celsius, −459.67° Fahrenheit) emits
thermal radiation. When the body emitting thermal radiation, the sun,
for example, has a higher temperature than whatever is in its path, the
resulting heat is absorbed – unless, as in the case of a Low-e coating,
something reflective is put in place. The Low-e coating keeps the heat from
being absorbed and emitted, instead reflecting it away from the glass. This
technology is related to the insulation used in spacecraft to keep heat from
accumulating on both reflective surfaces, inside and outside, but actually the
application of the technology is more complicated in windows and doors because
glass has such a high level of emissivity, thus posing the challenge of
maintaining low-e while letting in light.
Reflectivity
Through our discussion up to this point, you’ve probably
gathered that reflectivity refers to the property of objects to “bounce” heat
off, as opposed to absorbing and emitting it, and is inversely related to
emissivity. Metals, including aluminum, as in aluminum foil, are among the most
reflective substances, but most of us wouldn’t want to coat our windows and
door with foil. Instead, manufacturers of high-quality windows and doors apply coatings to reflect radiant energy, which helps to keep
heat on the same side of the glass it’s coming from – outside in summer, inside
in winter – while allowing light to pass through. This improves thermal
efficiency (insulating properties) because radiant heat stays where it belongs,
outside or inside depending on the season and the weather.
Opacity
Unless you want to keep out all light – for example, through
light-blocking blinds or shades that are always closed – there isn’t really a
way to entirely prevent at least a small fraction of thermal radiation (heat)
from coming in, or going out. Opacity refers to the “see-throughness” of
objects: for our purposes, glass in windows and doors. The more opaque an
object, the less its ability to let light through. It’s fairly obvious that
having no windows and doors could potentially allow you to nearly completely
insulate your home. Even a fully-sealed house will have some energy loss, if
the occupants ever leave, allowing for air transfer from in to out and
vice-versa. However, who wants to live like that, never seeing the sun shining,
kids playing in the yard, birds flying from branch to branch? For those of us
who like to come and go, who like to watch the world when we can’t always be
out in it, or choose not to because of less-than-ideal climate conditions,
coated glass gives us the best of all worlds. And the low-e coating is the best
technology we have to achieve that goal.
Putting it All Together
So what, in practical terms, does this mean to you as you
choose new windows and doors? It means you should look for windows and doors
with a Low-e coating or glaze, which can increase your home’s thermal
efficiency by as much as 100%. Glazed or coated glass is even more efficient
when it is combined with the additional technology of multi-paned glass with
argon between the panes. The combination of technologies maximizes the
insulating properties of your windows and doors. Not only will you conserve
energy – and, consequently saving money – but you will also enjoy a more
comfortable home.
In this article, we explain what "Low-e" means and
define the terms “thermal radiation”, “reflectivity”, and “opacity”,
particularly as they relate to glass in windows and doors. Finally, we have
tied it all together to show how this information can help you understand what
to look for when choosing new windows and
doors. If you found this article
helpful, please pass it on.
THE AUTHOR:
Gates Dearen is the co-owner of HomeRite Windows and Doors in
Jacksonville, Florida.
Owners Dearen and Richard Walden have been serving the
building products industry in Florida for
over 25 years. They know the products, the industry, the market and
what adds great value to a home. Their approach is a somewhat different
than others. They strive to match the homeowner with the
right windows and doors for their home and budget. They know that home
improvements projects can be a hassle. They strive to make the process pleasant
with first-rate, energy efficient products; affordable prices; and expert,
award-wining installers that employ the best practices and who
respect your home as if it were their own.
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