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Energy Performance In New Or Replacement Windows

by Gabby Hyman
Repair-Home Columnist

Selecting the right glass for a new home or for installation as a replacement window can present homeowners with overwhelming choices. Visit your local home improvement store and you'll find a dizzying assortment of windows organized by type, style, and energy efficiency.

In 2005, the National Fenestration Rating Council (NFRC) established a uniform rating system for windows, skylights, and doors to give consumers and contractors independent and unbiased energy performance information.

NFRC labels tell you how well a window performs across a wide range of hot and cold temperatures. Knowing how to read the label will help you strike a balance between cost and energy efficiency. NFRC ratings are optional, so buyer beware. If you're baffled about a particular product, ask a home improvement window specialist or contact a window contractor.

Navigating Window Rating Labels

The rating label should include the NFRC logo-the sun rising in the center of a windowframe. Going clockwise, you'll find panels that have rating values on the window's U-Factor, Solar Heat Gain Coefficient, Visible Transmittance, Air Leakage, and Condensation Resistance factors.

Here's what each panel evaluates:

  • U-Factor - The lower the U-factor value number, the better the window insulates against heat loss. Ratings typically fall between 0.20 and 1.20. If you have a large heating bill, you can save money by looking for a lower U-factor.
  • Solar Heat Gain Coefficient (SHGC) - SHGC measures how much heat penetrates your windows, evaluated in a fraction between 0 and 1. The lower the SHGC, the lower your cooling costs in hot weather. SHGC is vital in regions with hot summers and high consumer cooling costs.
  • Visible Transmittance (VT) - VT measures optical qualities of the window, indicating how much light comes into your living space, measured as a fraction between 0 and 1. If you like daylight views, you'll want to select windows with higher numbers.
  • Air Leakage - Air leakage ratings measure the amount of air moving through cracks in the window assembly package, which results in unwanted heat gain or heat loss, depending on the season. You'll want windows with a 3.0 or less rating.
  • Condensation Resistance (CR) - CR numbers measure the window's ability to resist interior moisture. High numbers (between 0 and 100) provide better resistance.


About the Author
Gabby Hyman has created online strategies and written content for Fortune 500 companies including eToys, GoTo.com, Siebel Systems, Microsoft Encarta, Avaya, and Nissan UK.