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,” or “be emitted” into the home. Perhaps an easier way to think of this is to consider a surface that has a high emissivity factor, like asphalt. If you’ve ever stepped on to 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
Solar heat-gain coefficient (SHGC) is a measurement of the amount of solar radiation that can pass through a window or door glass. There isn’t an overall standard for what’s best. Houses in geographic areas that experience more cold weather would be best outfitted with windows and doors with a higher SHGC, .30 to .60, to maximize the radiant heat from the sun. The reverse is also true. Homeowners in hot climates should look for windows and doors with lower SHGC values, under .27, to keep air conditioning costs down.
U-factor and R-value
The U-factor and R-value are similar, but U-factor measures the insulating properties of windows while R-value measures the insulating properties of other building materials like those used in constructing walls, floors, etc. The U-factor is one of the measurements used by The National Fenestration Ratings Council (NFRC) to determine the energy efficiency of windows and door glass. In determining the U-factor, the energy efficiency of the entire window is taken into account, including glazing, frame, and spacers. Sometimes U-factor is measured only at the center of the glass, taking into account only the glazing and the specific properties of the glass itself, exclusive of the frame and spacers. This measurement is called “center-of-glass U-factor”. For best energy efficiency, the center-of-glass U-factor should be lower than the overall U-factor, but in either case, for optimum efficiency, the U-factor should be under .30. Many double-paned windows achieve a U-factor of .27 or below, which is best in warmer climates.
Visible transmittance (VT) refers to the ability of windows and door glass to allow outdoor light to pass through. In the past, desirable sunlight gains were accompanied by undesirable (in the South, at least) heat gains. In other words, older types of glass were incapable of lettin
g in light without letting in radiant heat. With the latest glazing technologies, as well as the increased use of double-pane windows, glass can have a high VT while keeping a low SHGC. This is a big deal for homeowners in sunny, hot climates. It translates into energy savings, less fading of carpets and furniture, and greater quality of life. If you leave your house in the summer to go to work, you can turn down the air conditioner while you’re out, knowing that it won’t take long to cool it when you return. This isn’t the case if you have “leaky” windows and doors.
A few other things to check for
In addition to the information above, it’s important to look for the Energy Star symbol, given by the Environmental Protection Agency (EPA) to products that meet high standards for energy efficiency. Here are the criteria for a product to qualify for the Energy Star certification, from www.energystar.gov:
· Product categories must contribute significant energy savings nationwide.
· Qualified products must deliver the features and performance demanded by consumers, in addition to increased energy efficiency.
· If the qualified product costs more than a conventional, less-efficient counterpart, purchasers will recover their investment in increased energy efficiency through utility bill savings within a reasonable period of time.
· Energy efficiency can be achieved through broadly available, non-proprietary technologies offered by more than one manufacturer.
· Product energy consumption and performance can be measured and verified with testing.
· Labeling can effectively differentiate products and be visible for purchasers.
Another agency that reviews and rates products is The National Fenestration Ratings Council (NFRC), a nonprofit organization that tests windows and doors, among other building materials, and assigns a rating. The NFRC doesn’t make recommendations of what to buy. Their purpose is to provide information and let consumers know that a product performs as the manufacturer claims.
While the information above will help you to understand terminology, your best bet is always to choose a reputable windows and doors professional. He or she can guide you in your choice of the products that will work best for your home. Look for customer reviews and testimonials as well as before and after pictures. If you do your research on the product(s) and the vendor, you’ll likely reap the greatest rewards from your new windows and doors.
If you would like to receive an official factory authorized 30% discount coupon for your next window replacement purchase, print out this article with the coupon on it and bring it with you when visiting HomeRite.
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-winning installers that employ the best practices and
respect your home as if it were their own.