Thermal mass is a specific material's ability to absorb heat energy. A great deal of "heat energy" is needed to modify the temperature of a high density material such as concrete, therefore it is said to have "high thermal mass". Timber is said to have 'low thermal mass". Timber is a fairly lightweight material. Energy efficient homes consist of a simple design with maximum usage of natural energy sources to gain a high degree of comfort. What truly sets a passive design home apart from a standard tract home is thermal mass.

If solar heat is to be used when the sun is not shining, excess heat must be stored. Everyone has leaned against a sun-warmed brick or stone. It's warm and comfortable and takes a while to cool off. That's thermal mass. Thermal mass is simply a solid or liquid material that will absorb and store warmth and coolness until it is needed. Acting like a battery, thermal mass keeps the home comfortable by absorbing heat during summer and storing it for gradual release when it's cold.

In winter, thermal mass absorbs heat by direct sunlight. At night the process is reversed as thermal mass gives up heat, warming the room by radiation, convection and conduction. In the summer, thermal mass should be shaded so it draws the warmth from the surrounding air and cools the room. The greater the area of thermal mass, the greater its ability to store heat and maintain a uniform temperature.

How thermal mass is used in a passively designed home is the owner's choice. Thermal mass in the form of concrete, masonry or water has a much better storage capacity for heating and cooling than does the surrounding air. That's why thermal mass acts to prevent large changes of indoor temperature as the outdoor temperatures rise or fall.

The use of heavyweight construction materials with high thermal mass (for instance a concrete slab on the ground and insulated brick cavity walls) can reduce total heating and cooling requirements by up to 25% when compared to a home built of low thermal mass, e.g. lightweight construction materials of brick veneer with a timber floor.

The benefits of thermal mass depend on where the structure is located, how it is designed and how it is operated. But correctly done it can make a significant difference to comfort, and heating and cooling bills by averaging out highs and lows of temperature over a 24 hour period. In climates where there is a big difference between extreme heat and cold, thermal mass is very important, but less important, none-the-less has some benefit, in lower summer temperature areas. However in areas where access to the sun is limited, thermal mass could actually exaggerate the temperature peaks and troughs.

It's placement depends on what its main function will be. If cooling is a priority it must be protected from the summer sun and exposed to cooling night breezes. If heating is the main need, it must have access to winter sunshine. Generally thermal mass does not work in tropical climates, but is extremely effective in most other conditions.