Unit of heat resistance used as a measure of the effectiveness of insulation in stopping heat flow opposite of conductance. It is computed by determining the thermal conductance of a material expressed as thickness per unit area of the material required to maintain for one hour one unit temperature difference between its two sides. Dividing 1 by thermal conductance gives the material's R-value, and the higher it is the greater the material's heat insulation effectiveness and slower the heat flow through it. In metric units an R-value of 1 equals 0.17 square meter kelvin per watt, equal to 1.13 clo or 1.76 tog for clothing. A brick-and-mortar building typically has an R-value of 20 which can be enhanced to about 35 by using fiberglass insulation.