"Thermal mass" is all the stuff inside your home which the sun can heat: it includes everything from the walls and floor to furnishings to special construction materials designed to absorb and retain warmth. When you talk to professional installers, it is unlikely they will include minor incidentals in their definition of the term - they're mostly concerned with dedicated mass such as concrete slabs and Trombe walls.

There are two types of thermal mass to consider when you're looking at passive solar installations:

• Free (or incidental) thermal mass is all the mass contained in standard construction materials – drywall, door and window frames, doors, and even furniture. This is all stuff that is in every home and which absorbs an amount of the sun's energy even though it was not installed for that purpose.
• Intentional thermal mass is the opposite: it consists of building materials specifically chosen for their heat absorption and retention qualities. These materials are strategically used in locations where they will do the most good.

Good thermal mass comprises most solid, dense materials, and especially those of dark color. Brick, concrete, tile, and earth are all forms of good thermal mass which are used in virtually all construction. You'll notice that holistic home design once again comes into play here, as all these materials serve other purposes – fireplaces, masonry heaters, kitchen flooring, and so on. The trick, as always, is to choose and use materials that give you as many advantages as possible, both individually and from the combination of them all.

How thermal mass works

You're probably familiar with how thermal mass absorbs and retains heat, but we'll cover the basics here for completeness. When the sun hits a surface, some of its energy is converted to heat and either stored in the material or radiated out into the surroundings. The amount of heat a material can hold is determined by a variety of factors, the most important of which is its density: heavy, solid substances can store more heat than light, flimsy ones. Dark colors absorb heat better than light colors, so dark walls and floors will warm up quicker than light-colored alternatives.

But sunlight isn't the only heat transfer medium: walls and floors will absorb heat from anything that touches them and which is warmer than they are. That can be air passing across the surface, water running in pipes installed behind the surface or even someone walking barefoot on a cold floor.

The heat which is absorbed obeys the law of physics which states that everything tries to maintain an equal temperature. If the surroundings are cooler than the thermal mass, heat is radiated back out; if not, the heat is sucked deeper into the cool heart of the solid material. Over time, the whole of the thermal mass can warm up, right down to the deepest point inside it. Only when the exterior (room) temperature falls below the temperature of the thermal mass is the heat gently released in order to establish equilibrium.

All of this happens passively. There are no blowers, pumps