Describes the rate of Thermal energy passing through a material due to conduction, convection, and radiation under specific environmental conditions. It is calculated using material thermal conductance and surface emissivity values which are intrinsically measured. Lower values describe lower rates of heat energy transmitted through a material and hence improved insulation values.
For glazed areas, the surface emissivity of glass can be dramatically reduced by high performance coatings and this is a major factor in reducing this value.
U-Value is expressed in units of Btu/hr ft² °F, K-Value in W/m² °C and different standardized conditions are used for these calculations.
To convert imperial to metric values multiply by 5.6783