n. A mass that holds heat.
a. Having the ability to hold heat.
Sounds boring doesn't it? It's not though; it's actually quite cool. While the concept of thermal mass is used in a number of fields, the contexts you are most likely to hear it in are those of architecture and green building. It is becoming common to look at the thermal mass of a building as a useful and energy efficient feature, because buildings with high thermal mass often manage heat better than 'traditional' buildings.
A house with a high thermal mass, for example with one built with thick brick or clay walls, will act as a storage battery for heat, soaking up heat during the day and slowly releasing it when the ambient temperature cools back down. This means that the walls of an adobe house can absorb the heat of the sun during the day, keeping the interior of the house cool, and then slowly release that heat throughout the night, keeping the occupants warm.
In the in cold climates thermal mass may be used in a different way. thermal mass heaters (AKA masonry heaters) can be built that will allow for hot fires to dump their heat into thermal masses such as walls, floors, and chimneys. While a traditional fireplace will give off heat only while the fire is burning, a thermal mass heater can soak up the heat and slowly release it for hours. This means that stoking the fire once a day can provide heat for 12-24 hours. Stoves and ovens designed as thermal mass heaters are generally highly efficient, burning very hot and very cleanly, and are built with long, often labyrinthine chimneys that allow the masonry to soak up nearly all the heat from the fire. The chimneys are often built through the floor (longer chimneys are actually more efficient, and can draw downwards or sideways without problems), the walls, and even built-in furniture such as benches and bed platforms. The downside is that it can take a long time for the heat to enter the living space, as it will travel slowly through the thermal mass.
Thermal masses are often built to collect the heat of the sun, and are usually built facing South (in the Northern hemisphere), and often have special features such as trombe walls to help them collect heat more effectively.
While thermal masses are good for surfaces next to a heat source, they aren't so good in other areas. An exterior wall that is often in shadow (such as a Northern-facing wall) will not be able to soak up heat from anywhere but inside the house, and will slowly and constantly 'leak' heat to the outside. In this case you should plan on having some good insulation on the wall.
Thermal masses are not considered effective insulation because they have a low r-value (they do not stop heat flow). This is one reason some building methods, such as cob and adobe, are having a hard time being accepted under current building codes, which usually require certain r-values. While a thermal mass may provide better temperature regulation through passive heating and cooling, it just isn't accounted for in most codes.
Of course, walls aren't the only form of thermal mass. Water is one of the most efficient thermal masses, so if you have a water tank you can paint it black and enjoy passive solar heating and warm water at the same time. This is particularly effective if you are subsisting on rainwater or other collection methods that require large tanks.
The Earth is also a very large thermal mass, which stays at a comparatively stable temperature year-round. In much of America and Europe the ground stays at about 50 degrees Fahrenheit (10 degrees Celsius) throughout the entire year. While we usually want our houses warmer than that, this can be useful in particularly hot or cold climates, keeping houses cool in the summer or aiding heaters in the winter. We can also heat the ground through passive solar heating, allowing the heat to be released during the colder night; some Earthships use this principle. If your house is carefully planned, some designers claim that summer heat can even be stored for the winter months.
For Wintergreen: An Earth Quest