or other equipment: the thermal mass simply absorbs and radiates heat in an attempt to maintain a stable temperature.

Interior and exterior thermal mass

No matter where thermal mass is located, it will absorb and retain heat. Internal thermal mass – walls and floors inside the home, for example – are best located in the path of direct sunlight so that they can absorb as much energy as possible. External thermal mass will get hit by the sun regardless (unless it's shaded by obstructions).

In colder climates, it is best to favor internal thermal mass. You want the sun to penetrate as deeply into the building and warm as many dark, dense surfaces as possible, so that the maximum amount of warmth is stored close to the heart of the home. This heat is then held within the building by envelope insulation in the floors, ceilings, external walls, and foundations.

In hot, dry climates, it is best to balance internal and external thermal mass. The sun's heat warms the outside of the whole building a lot more than in cold regions and that heat is passed to the interior through the exterior mass. Interior thermal mass acts in the same way as in a cold climate, of course.