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Using Window Rating Systems When Selecting Replacement Windows

April 30, 2010

Replacement window ratings can be confusing but are important for selecting the right windows for your home. Here's a rundown on window rating systems you'll likely encounter:

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R-value (aka R-factor): Established by the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRA), this rating measures a window's ability to resist heat. A higher R-value indicates higher heat resistance. Factors influencing R-value include:

  • Glazing material
  • Number of layers of glass
  • Amount of space between glass layers
  • Thermal resistance level of other window materials including spacers and frames
  • Proper installation, i. e., no gaps or air leakage

Typical R-values range between .9 and 3.0.

U-value: Ranging between 1.1 and 0.3, this value measures how heat enters or exits through a closed window. Well insulated windows have a low U-value and a high R-value.

Solar Heat Gain Coefficient (SHGC): This rating measures how much solar heat penetrates through a window. This rating is based on a scale of 0 to 1.0. The lower the SHGC rating, the less solar heat is radiated through a window.

Condensation Resistance (CR): This rating measures how effectively a window resists condensation between glass layers. The higher the CR rating, the better the condensation resistance. Condensation can vary according to temperature and environmental conditions; no replacement window can entirely prevent condensation.

Visual Transmittance (VT): This rating measures light transmission through glass on a scale of 0 to 1, with higher numbers indicating more light transmission. VT ratings take into account window frame size, but other variables can affect individual perception of light transmission.

Not all replacement windows display all of these ratings. Consulting with window dealers can help you find the combination of ratings matching your preferences.


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