thermal mass

This term is primarily used in architecture, sustainable design, and thermodynamics to describe the heat-storage capacity of heavy materials. It carries a technical, professional register and is most frequently encountered in discussions regarding passive solar heating and energy efficiency.

Because it refers to a physical property of a material or a structural element, it is treated as an uncountable noun. You would not refer to multiple thermal masses in a house, but rather to the amount of thermal mass present or the materials providing the thermal mass.