U Factor

The U-factor, also known as thermal transmittance, is the overall heat transfer coefficient through a particular body or assembly, including its boundary films. Note that the U-factor for a construction assembly, including fenestration, includes the interior and exterior film coefficients (the boundary films referenced above). For characterization of fenestration products, the U-factor is calculated for the whole product, including the effect of the frame (center of glass, edge of glass, frame). The U-factor is the reciprocal of the R-value.