Measure of a material’s heat-conducting properties. It is used in the building industry to compare the efficiency of insulating products, a good insulator having a low U-value. The U-value of a material is defined as the rate at which heat is conducted through it per unit surface area per unit temperature-difference between its two sides; it is measured in watts per square meter per kelvin (W m-2 K-1). In equation terms, it may be calculated as the rate of loss of heat/surface area × temperature difference.