Thermal mass is what really differentiates  a passive solar or zero energy house from a regular house

To use the heat of the sun after it has set, we have to find a way to store this energy as long as the sun shines. This storage is done in the thermal mass. Your ability to store and re-use this collected heat after sunset will be a big saver on your electric bill.

Rule_1: Heat storage in thermal mass

You probably know that when you touch a wall that has been in the sun for a while, that it´s hot and needs time to lose its heat. This wall is called a thermal mass. The thermal mass normally consists materials that weigh a lot and are capable of storing heat and transmitting this heat when the surrounding temperature cooled off. Thermal mass can be a wall or even a barrel with water. Storing heat in the thermal mass and transmitting it later when needed allows for a room to even out the outdoor temperature differences between the time the sun is shining and when not.  With the right balance between the amount of glazing to collect the sun´s heat and the correct capacity of storing the heat in the thermal mass, you can keep the in-house temperature between 68 and 70 degrees F.

The form in which you´ll find thermal mass varies from brick, tile or thick concrete floors, fireplaces or interior walls made of bricks or stones or even barrels with water.

Rule_2: Heat transfer

In wintertime the thermal mass collects the heat of the sun and during the night it transmits this heat to the room. This is done by three thermal processes:

• Radiation: this is the heat that you notice when you´re looking into a fire. One side of your face is hot, the other part cold. A passive house takes up heat through radiation by the direct incoming sunlight.
• Convection: the air close to the thermal wall is warmed up and will rise. Colder air will replace the rising hot air and will be heated as well by the thermal wall. The cooled off air at the ceiling will come down again to be heated. By this you get the circulation, or convection, of air.
• Conduction. Conduction is when the heat is transmitted between connecting materials. Heat wants to escape from the heated space to the outside following the path with least resistance. And heat transfer spot is called a ´thermal bridge´. An important loss of heat through outside walls by poorly insulated thermal bridges occurs through this principle of conduction.

Rule_3: Shade your thermal mass

In summertime make sure that your thermal mass is shaded so that it cannot receive direct sunlight. At the same time it will absorb the heat from the air that surrounds it, which will cool the room.

To save on your electric bill, you not only want to get lots of sunlight into your house, but also want to find ways to store and transmit this heat when the sun has set. The principle of thermal mass allows you to make efficient use of heat that can be collected from the sun during the day. Thermal mass therefore is the most import element of a truly passive solar house design.