FAQ Knowledge Articles

U-value (U-factor) is the rate of heat loss of a window.  The lower the u-factor the better the window's insulating properties.  This is linked to the more common material insulation rating R-value.  1/(U-value) = R-value
Ex: If U = .33, then R-value = 1/.33 = 3
      If U = .25, then R-value = 1/.25 = 4
Actual wall R-value will differ from U/R-value of the window.  The percent of the window to wall area and wall insulation level will be contributing factors in the overall wall R-values as shown in the below chart.
User-added image
NOTE: U-Values above may not be available.
www.efficientwindows.org/ufactor.php