Heat Capacity (HC) is a term used by many energy efficiency standards to quantify the amount of thermal mass in a wall or the ability of the wall to store heat. Heat capacity is the heat capacity per square foot of wall area (Btu/ft²-°F) and includes all layers in a wall. For a single layer wall heat capacity is calculated by multiplying the density of the material times its thickness (in feet) times the specific heat of the material. Heat capacity for a multi layered wall is defined by the addition of all layers.
The heat capacity of a wall is like the specific heat, in that it defines the energy required to raise the wall temperature one degree, but it is expressed in terms of wall surface area rather than pound of material. Heat capacity, then, is the amount of energy required to raise the temperature of one square foot of wall by one degree Fahrenheit, or Btu/ft²-F. It is found by multiplying the wall weight by the specific heat.
Heat capacity is sometimes called "areal" heat capacity, in reference to area measurement, to distinguish it from "volumetric" heat capacity, which is the heat capacity per cubic foot, or Btu/ft3-F. In this sense, heat capacity is a special term, but it is widely used in energy codes.