What Do Replacement Window Ratings Mean?

August 02, 2010

When shopping for replacement windows, look for the NFRC (National Fenestration Ratings Council) label, which contains ratings of window performance. The NFRC provides energy performance ratings for windows, doors and skylights. NFRC ratings include how the entire window assembly including glazing and the frame, perform. Here's what their ratings mean.

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  • U factor: or U value: This rating measures how well a window prevents heat flowing through it. This rating scale ranges from .20 to 1.20. Look for the lowest rating, as this means that a window allows less heat to flow through.
  • Solar Heat Gain Co-efficient (SHGC): This rating measures how well a window blocks heat caused by sunlight. The SHGC rating scale is between 0 and 1, with lower ratings indicating less heat transmitted through the window. This rating is important for windows exposed to direct sun.
  • Visible Transmittance (VT): This rating measures how much light penetrates through a window. This rating scale ranges between 0 and 1. The higher the rating, the more light comes through.
  • Air Leakage (AL): Used for measuring how much air leaks through a window assembly, this rating is useful for selecting windows that conserve energy. The lower the AL rating, the less air potentially leaks through.
  • Condensation Resistance (CR): Condensation trapped between glass layers is frustrating; the CR rating measures a window's ability to resist condensation on its interior surfaces. Rated between 0 and 100, a higher rating indicates that a window is less likely to form condensation.

Compare and use window ratings as tools for choosing new windows based on your needs. Manufacturers are not required to use the AL and CR ratings; if this information is important to you, ask window dealers to direct you to products that include these window replacement ratings.

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