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A measure of heat transmission through a building part or a given thickness of insulating material, expressed as the number of British thermal units that will flow in 1 hour through 1 square foot of the structure or material from air to air with a temperature differential of 1°F.
A measure of the amount of heat that flows in or out of a substance under constant conditions when there is a 1-degree difference between the air within and outside a building. U-values are used in determining the performance of a glazing system or window assembly.
thermal transmittance, U-value
The time rate of heat flow per unit area under steady conditions from the fluid on the warm side of a barrier to the fluid on the cold side, per unit temperature difference between the two fluids.