(pronounced r VAL-yoo)

Otherwise known as Resistance-value, this term is a measure of thermal resistance – i.e., resistance to heat transfer or flow – used in the construction industry. The bigger the R-value number, then the more effective the insulation will be in holding heat in a structure and keeping heat out of a structure, thereby lowering its operational energy costs. R-value is the reciprocal of U-value, which describes how well an element conducts heat.