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Words for Windows

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• The U-Value is a measure of the thermal conductivity of a window: The lower the U-value, the better a window is at limiting heat losses. A single-glazed window has a U-value of about 6 W/m 2-K, while triple-glazed windows have U-values between 1 and 2.
  
  
• The shading coefficient describes a window's ability to transmit solar heat gain, relative to the transmittance of a 3-mm, clear single pane of glass. The shading coefficient is given as a unitless value between 0 and 1; the lower the SC the less solar heat it transmits. (The shading coefficient is being replaced by the "solar heat gain coefficient" which gives the fraction of incident energy that enters the space (with a value between 0 and 1).
  
  
• Visible transmittance is the percentage of visible light striking the glass that penetrates to the interior, expressed as a number between 0 and 1.
  
  
• Spectral selectivity is a window's ability to transmit visible light while reducing solar heat gain, and is expressed by a number between 0 and 1.
  
  
• The coolness factor is the ratio of the visible transmittance to the shading coefficient. If the value is greater than 1.0, the glass is considered "selective" and will offer better performance in a hot climate than a similar glazing with a lower coolness factor.

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