A measurement of the rate of heat flow through a window. The lower the number the better the insulating performance.
a calculated value used to further calculate the rate of heat flow through a structure, e.g. the heat conducted through a wall, roof or window
Heat transfer coefficient of material.
The measurement used in determining the ability of different structural components (such as windows) to conduct heat. The "U" Value of a window is measured by the number of Btu's that will pass through each square foot of area per degree of temperature difference from one side of the window to the other. "U" Values can tell you how well your windows will hold in your heated or cooled air. The lower the number the better.
A measure of the rate of thermal transmittance through glass, which is directly related to the weather conditions. Usually determined using conditions established by ASHRAE (American Society of Heating, Refrigeration and Air Conditioning Engineers). The lower the U-value, the lower the thermal transmittance and hence the better the insulation.