R-value
Quantitative measure of resistance to heat flow or conductivity, the reciprocal of U-factor. The units for R-value are ft2 ̊F hr/Btu (English) or m2 ̊K hr/W (SI or metric). While many in the building community consider R-value to be the primary or paramount indicator of energy efficiency, it only deals conduction, one of three modes of heat flow, (the other two being convection and radiation). As an example of the context in to which R-value should be placed, 25% to 40% of a typical home's energy use can be attributed to air infiltration.
