Transmittance, Thermal (U-Value)

The combined thermal value of all materials in a insulation system, air space, or surface air film. Heat transmission in unit time is measured as the unit temperature difference exhibited on all sides of an insulation system. I-P units are Btu / (hr – sq ft – deg F temperature difference). SI units are W / (sq m – deg C temperature difference. This rate of heat transmission is also known as the overall coefficient of heat transfer.