U value
engineering UK For building materials, a measure of thermal conductivity per unit thickness, being B.t.u. per square foot per hour per degree Fahrenheit; a U value of 1 = 5.678 3~ W·m-2·K-1. A value less than 1 has traditionally been seen as appropriate for energy efficiency; traditional brick cavity walls have values about 0.3, but many roofs exceed 1. For the greater efficiency wanted in the modern world, a value of 1 in metric wattage terms is more the target. It is the reciprocal, for a given time and area, of R value.
[Von Straaten J. F. Thermal Performance of Buildings (London: Elsevier, 1967)]
