UPDATED on December 4, 2013 with a citation of recent research findings.
What’s the deal with thermal mass? Since manufacturers of materials that incorporate concrete often exaggerate the benefits of thermal mass, it’s easy to get cynical and conclude that the buzz around thermal mass is all hype. But in many climates, it’s actually useful to have a lot of thermal mass inside your house. Just keep in mind that thermal mass may not be as beneficial as its boosters pretend.
Thermal mass is a solid or liquid material that can store heat. Most of the objects inside your house can be considered thermal mass, including plaster, furniture, books, and canned tomato soup.
The specific heat capacity of building materials varies. In general, denser building materials have a higher specific heat capacity per unit of volume than less dense materials, which is why concrete, stone, and gypsum wallboard are more likely to be used to provide extra thermal mass than wood.
Three analogies: cistern, frying pan, truck
A building with lots of thermal mass on the interior side of the insulation may have lower energy bills than one without as much thermal mass, for reasons I’ll explain soon. But it’s important to point out that thermal mass can’t heat or cool your house. It’s just plain old concrete. To heat and cool your house, you still need HVAC equipment.
Here are some analogies: