U-Value
The amount of heat energy (BTU's) transferred through one square foot of a specific glazing system for each 1°F temperature difference between the indoor and outdoor air. The overall coefficient of heat transfer by conduction equals the reciprocal of R-Value (U = l / R). The lower the U-Value, the better insulating qualities of the glazing system. U-values are measured under standard conditions given by either the ASHRAE Summer Conditions or the AIMCAL Winter Median Conditions.