Definition:

U-Value is the heat transfer coefficient of a material or an assembly of materials. It is measured in terms of Btu per hour, per square foot of area, per degree of temperature difference across the material. The reciprocal of the U-value is the thermal resistance or R-Value. The lower the U-value number, the greater the heat transfer resistance (insulating) characteristics of the material or assembly of materials.

Related:

• R-Value