What does the term “R-value” mean?

R-value is a measure of how well a material resists the passage of heat. The higher the R-value, the slower the rate of heat transfer through the insulating material and the more effective insulation is in keeping the home warm in winter and cool in summer. Insulation should always be judged by R-value rather than inches, as different insulation materials have different R-values per inch of thickness.