- 1. see roentgen2. see rayleigh3. a commercial unit used to measure the effectiveness of thermal insulation. A thermal insulator is a material, manufactured in sheets, that resists conducting heat energy. Its thermal conductance is measured, in traditional units, in Btu's of energy conducted times inches of thickness per hour of time per square foot of area per Fahrenheit degree of temperature difference between the two sides of the material. The R value of the insulator is defined to be 1 divided by the thermal conductance per inch. This means R is an abbreviation for the complex unit combination hr·ft2·°F/Btu. In SI units, an R value of 1 equals 0.17611 square meter kelvins per watt (m2·K/W). In clothing insulation units this is about 1.136 clo or 1.7611 tog. Usually the symbol R is placed before the numerical value, as in R15 or R-15.See also RSI.
Dictionary of units of measurement. 2015.