R-Value

A measure of the overall resistance of a glazing material to heat transmission due to the difference in air temperature on either side of the material. A high-R-value wall or ceiling has a greater resistance to heat flow and a higher insulating value than one with a low R-value. New home walls are usually insulated with 4" of batt insulation with an R value of R-13, and a ceiling insulation of R-30.

The R-value is the inverse of the U-value (R = 1/U) and is expressed in units of hr-sq ft-°F/Btu.