R-value is a term that is used in the construction and housing industry to measure the amount of thermal resistance that a particular material has. R-value is most commonly used as a way to compare the effectiveness of a range of different insulation materials. Products that have a higher R-value enable higher energy efficiency because they slow down the transfer of hot and cold air between a home and the outdoors. The United States Department of Energy recommends a variety of different R-values for property owners depending on their geographical location, the climate and the area's average utility bills. The R-value of a particular insulation is often based on its density and the thickness of the product once it is installed. For example, because thicker amounts of the same insulation have a higher R-value, some builders use 2x6 lumber on the exterior walls instead of thinner 2x4 studs.