Anatomy of an Efficient Window

Amanda Bell
Written by Amanda Bell
Updated February 12, 2016
wood replacement windows in living area
Understand what comprises an energy-efficient window before having one installed in your home. (Photo courtesy of Lindus Construction)

Understand what comprises an energy-efficient window before having one installed in your home.

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Are you throwing money out the window in the form of excessive heating and cooling bills? According to the U.S. Department of Energy, you might be: About a third of a home’s heat loss occurs through windows and doors.

You can minimize heat loss and give your home an efficiency boost by replacing old windows with new ones that exceed Energy Star specifications. While energy-efficient windows often cost more than traditional products, the payoff comes with lower utility bills. Consider replacing your windows with ones containing any of the following energy-efficient components.

Multiple panes

Multiple panes provide more insulation than windows with a single pane of glass. Usually available in double- or triple-pane configurations, the sealed layer or layers of air between the panes help prevent heat transfer or escape.

Low-E coating

This coating covering the panes of glass reduces heat transfer. That means it can prevent heat from entering a home in warm climates or stop heat from leaving a home through a window in cooler climates.

Gas fill

Gas fill like argon or krypton can help insulate windows better than air because they offer more thermal resistance. These inert, non-toxic gases don’t react to other substances, meaning they don’t pose any home health hazards if they leak over time.

Framing material

Options include wood, fiberglass and vinyl. Highly durable vinyl frames offer insulated cavities, which can give your home’s energy efficiency a boost. Fiberglass frames have a higher heat transfer resistance than wood or vinyl but are more expensive. While naturally efficient, wood frames tend to be high-maintenance and pricey.

What’s on the label?

Products carrying the National Fenestration Rating Council (NFRC) label have been tested to ensure their energy efficiency claims are accurate. Below are the four types of numbers found on these labels that you can use to evaluate a product:


Measures how much heat from inside a room can escape. This number typically falls between 0.2 and 1.2. A lower number means a window has lower potential for wasted heating expenses.

Solar Heat Gain Coefficient (SHGC)

Gauges how much heat from outside can enter a room. Expressed as a number between 0 and 1, the lower the number, the less likely a window will waste cool air.

Air Leakage (AL)

Represents how much air can enter a room through a window. This number typically falls between 0.1 and 0.3. The lower the number, the lower potential there is that draft will come through a window.

Visible Transmittance (VT)

Quantifies how much light can come into a room. Expressed as a number between 0 and 1, the higher the number, the more natural light can come through.

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