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"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.
A good passive solar home design maintains a balance between solar and non-solar windows. Solar windows are those which face the sun, on the side of the house pointing true south; non-solar windows are those on the other sides.
Thermal mass is everything inside a home that absorbs, retains and later emits heat, effectively providing free heating. Like most things in home construction, there are many different approaches to its use.
Let's step away from the "white picket fence" ideal of home design for a moment to consider one of the other extremes: log cabins. Some people say log homes and cabins are extremely efficient, while others claim that they cannot compare with the advantages of modern technology. So which is true?
As with most "which is better?" questions, the answer lies somewhere in the middle ground.
At first glance, you might think that it's impossible to have too much or too little thermal mass. After all, if there's loads of it, your home will store more heat; if there's very little, you'll still get the benefits of what you do have.
But it's not as simple as that.
In an ideal passive solar world, homes would be built as a long string of single rooms, so that every room would be heated by the sun and there would be no need to transport warmth from warmer to cooler rooms. In practical terms, long houses are rarely appropriate: traffic flow inside such buildings is often difficult and housing lots in towns and cities tend to be square or rectangular, rather than extended oblongs.