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Understanding Window Glass Ratings

December 22, 2009

Window glass ratings measure several factors important for selecting replacement windows. The National Fenestration Council (NFRC) provides ratings measuring several aspects of window performance. All energy star windows include NFRC ratings:

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Window Glass Ratings: Measuring Heat and Air Flow

  • U Factor: This rating measures non-solar heat flow through a window. NFRC ratings for U factor consider the entire window as opposed to only window glass. The lower the U factor, the more energy efficient the window.
  • Air leakage: measures levels of air leakage. Choose windows with a low air leakage factor.
  • Solar heat gain co-efficient (SHGC): This rating is important if you want stronger shading qualities, for example, if your home is exposed to intense desert sunlight. The lower the SHGC rating, the more effectively the window blocks solar heat transmittance.

Let the Sunshine in (or Keep it Out)

There are two measurements for visible light transmission, which measure how much visible light enters your home through a specific window. These ratings can help in selecting windows that allow more natural sun light into your home, or replacement windows capable of reducing glare.

  • Visible transmittance (VT): This rating measures how much natural light penetrates a window. If you want more daylight indoors, select replacement windows with a higher VT rating.
  • Light-to-solar gain ratio (LSG): This rating measures the efficiency of window glass in transmitting light while blocking heat gain. Higher ratings indicate that windows can transmit more light without accompanying increases in heat gains.

When shopping for replacement windows, make a list of factors important to you. Matching window glass ratings to your needs can help ensure that your new windows meet your expectations.


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