A key element in the heating and cooling of any structure is the relative tendency of its skin to resist heat flow. An exterior resistant to heat transfer effectively provides a layer of insulation which more heat conductive materials do not. Masonry walls have a more favorable rate of heat transfer because of their greater heat storage capacity, which is sometimes referred to as thermal mass, or capacity insulation. Builders used this characteristic in their designs for centuries prior to the development of modern air conditioning methods, which are capable of overcoming less efficient designs at the cost of increased fuel use.