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 in it are 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 a high thermal mass often manage heat better than 'traditional' buildings. A house 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 soak up the heat of the sun, keeping the interior of the house cool, and then slowly release the heat throughout the night, keeping the occupants warm.

In the winter thermal mass heaters (AKA masonry heaters) allow for hot fires to dump their heat into walls, floors, and chimneys with high thermal masses. While a traditional fireplace will give off heat only while the fire is burning, a thermal mass can soak up a burst of 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 super 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), walls, built-in benches and 'bed frames', and anything else that can beneficially be warmed. The downside is that it takes a long time for the heat to enter the living spaces, as it will travel very slowly through the thermal mass.

Thermal masses are often built to collect the heat of the sun, and 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.

Thermal masses are not considered effective insulation because they have a low r-value (do not stop or drastically slow heat flow). This is one reason some building methods, such as cob and adobe, are having a hard time being accepted by building inspectors. The building code usually requires certain r-values, and 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, and if you are subsisting on rainwater you can paint your water tank black and enjoy passive solar heating and warm water.

The ground is a very large thermal mass, which stays at a comparatively cool 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. We can also heat the ground through passive solar heating, allowing the heat to be released during the colder night. If your design is carefully planned, some designers claim that summer heat can even be stored for the winter months.

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