Labels on Energy Star Windows?

Interpreting the Label on Energy Star Windows

by Corey Palmer

Listening to a salesman present the benefits of Energy Star windows may leave you confused; reading the energy labels on windows at a store may make you walk; and talking with your builder may make you want to stop renovation. If you experienced one of these scenarios, then you are in need of some orientation. The following are terms usually used in energy performance ratings, such as in the ENERGY STAR labels.


U-Factor : This is the value used to determine insulation of the windows. Heat can be transferred through the windows in non-solar ways, such as through radiation, convection, conduction and air leakage. With a low U-factor, you can conclude that the windows have high insulating values to keep heat or cold at bay.


R-Values : The opposite of U-factor, R-values represent the level of heat resistance of the energy efficient windows against heat transfer. Look for windows with high R-values as it also indicates low U-factor, which means high insulating value.

Low-E Coatings : Low-E (low emissivity) coatings are transparent metallic oxides that provides extra protection from the sun’s heat. When combined with gas fillings, the amount of heat reflected by the coatings can be compared to having an additional window glazing.


Gas Fillings : In the vacuum space between a double or triple glazed window, gas fillings such as Argon and Krypton can be filled to add another layer of insulation to the windows.


Visible Transmittance (VT) : In many cooling climates, views of the outdoors are often exchanged for window shades or glass tints to prevent heating up the house or building. While tinted windows can still provide a view of the outdoors, the view is much less appealing than when a clear glass window is used. The amount of daylight admitted and visibility of the outdoors are rated by the Visible Transmittance value.


Solar Heat Gain Coefficient (SHGC) : In the summer, solar heat gain can escalate and leave the interior overheated. A window that has a low solar heat gain coefficient is ideal in this season as it blocks a fraction of heat that can be transferred through the windows.


Glazing, in layman’s terms refers to the glass panes of a window. A single-glazed window only has one pane of glass and two panes of glass for a double-glazed window. Glazing can reduce heat transfer and when filled with gas fillings and coated with Low-E, you will have a good performing window.


These terms are found on the labels provided by the National Fenestration Rating Council (NFRC) and should be your guide when choosing energy efficient windows.


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