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. In the Dallas/Ft. Worth area, the Department of Energy suggests R-values of 49 for attics and 25 for floors, in both new construction and existing homes.