Inertia: beware of popular belief
Inertia (the ability of a material to store and then release heat) of course contributes to the energy performance of a building envelope.
But it is the inertia of the entire wall, comprising building material and insulation, that is considered. The inertia class of the insulator is an element of the DH calculation but not the main parameter. Stone and concrete, for example, have higher absorption capacity than insulation, and their contribution to overall inertia is greater than that of insulation.