U Value

Published February 2000

Heat transfer coefficient. A measure of the heat transmissivity of a particular insulation medium or combination of media. More specifically, the heat that is lost (or gained) through a unit area of the insulation medium per degree of temperature difference between the inside and outside surfaces. Units: Btu/hr/ft2øF, or W/ m2øC. Heat loss or gain of an entire wall system can be determined by multiplying the U-value times the total wall area, times the difference between the inside and outside temperatures.