SAP and thermal mass. SAP 2009 uses thermal mass in calculating the heating and cooling load of the building. SAP uses the kappa (k) value to determine thermal mass. 'k' is the measure of the heat capacity per unit area in kJ/m 2 K of the 'thermally active' part of the construction element:. k = 106 Σ i p i c i d i. p i = the density of the layer 'i' in the construction (kg/m 3)