R-value is a measure of thermal resistance to heat flow, or how well
the insulation slows heat flow. The larger the R-value, the more
effective the insulation and the less heat flow. Each wall, ceiling,
and crawl space in the home may have a different R-value. The U.S.
Department of Energy Web site contains a computer model that suggests
R-values while taking into account specific home factors, such as
efficiency of air conditioning equipment.