How green windows can cut energy bills

March 01, 2012

The higher the rates of heat flux in your home, the poorer your protection against energy loss and high utility bills. Heat -- or thermal -- flux rates measure the rate at which air is conducted through the window. In some cases, a single-paned window is like having nothing inside the opening at all. Here's the scoop on the energy efficiency ratings you will see when shopping for replacement windows.

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According to Allen Liu at the University of California at San Diego, the thermal conductivity of a single pane of glass is over 40 times greater than open air. In his study, Liu concluded that double-pane windows can lower energy costs between 15 percent and 30 percent while reducing street noise and increasing resale value.

Shopping for double pane windows for home

National Geographic writer Angela Brady recommends that homeowners buy double pane windows with argon or krypton gas fills to maximize on energy savings and reduce heat flux rates. To further improve window insulation, Brady suggests you choose the optional low-emittance coatings.

According to the Efficient Windows Collaborative, you should choose the right types of coating or glazing for your climate. You may have to strike a balance of features in choosing windows for your home. Remember, high solar-gain glazing is best for cold conditions, while the low solar-gain glazing can keep out heat in summer.

Comparing energy efficient window properties

Get acquainted with the National Fenestration Rating Council (NFRC) label on the window products in your home improvement store or factory showroom. The label has five boxes or divisions in it. The top two boxes are for energy performance ratings; the lower ones list "additional" performance ratings.

The top two boxes show the ratings for U-Factor and the Solar Heat Gain Co-Efficient (SHGC). When comparing products, remember that the lower the number is for the U-factor, the greater the window's ability to hold internal heat. The SHGC measures how well the window can keep the heat from outside sunlight at bay. Windows with lower SHGC ratings offer the greatest defense against unwanted heat gain.

The lower panes of the label measure Visible Transmittance (VT) and Air Leakage. With VT, the higher the number, the greater the amount of light that penetrates the windows. Depending on your interiors, you may wish to shop for low VT ratings to best protect your carpet, drapes and furniture. Air Leakage numbers map the amount of air movement between parts of the window, frame and assembly. The lower the number, the tighter the thermal envelope of your home.

Another common rating, posted in the lower left hand section of the label, measures the product's Condensation Resistance (CR). Condensation between the panes or frame assembly can create an opaque window and, in worst-case scenarios, mildew and rotting frames. The higher the CR number, the greater the resistance.

Now you know how to shop. With energy bills on the rise, you can fight back with efficient windows. Double up on the panes and fills, and choose optional coatings that deliver the best performance for your climate.

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