U-Value measures how well a window or glass wall holds heated or cooled air inside the building. More specifically, it is the measure of heat energy transferred from one side of the glass unit to the other, measured by the amount of heat transferred through 1 sq. ft. of glazing for each 1° F temperature difference between the indoor and outdoor air. The lower the U-Value, the higher the thermal performance of the glass wall. U-Value can be improved by insulation, lamination, and specialty thermal coatings.