U-Value: A measure of air-to-air heat transmission (loss or gain) due to thermal conductance, and the difference in indoor and outdoor temperatures. As the U-Value decreases, so does the amount of heat that is transferred through the glazing material. The lower the U-Value, the more restrictive the fenestration product is to heat transfer (Reciprocal of the R-Value). This is a critical function in that it makes it possible to strike an optimum balance between economy and ecology. The lower the U-Value, the more efficient the insulation.