Thermal mass is vital for effective passive solar design. It makes it possible to store the heat from the sun shining into the house during the day in heavy materials like concrete, brick, and stone. The effect of the thermal mass is to even out the temperature fluctuations. In winter it makes the interior slower to heat up when the sun is coming in and much slower to cool down when the sun goes down (or behind clouds). In summer the eave overhang prevents the sun from coming in so the concrete floor helps to keep the building cool. By opening the windows at night the whole house can be cooled down. During the day in summer the windows and blinds are kept closed to prevent heat from getting in and the place will stay nice and cool.
To be effective the thermal mass must be INSIDE the thermal boundary (insulation and air sealing layer). Bricks on the outside of a building do no good for energy conservation. And solid brick, masonry, stone, or earth walls do not have adequate insulation. There has to be an insulating layer on the outside of the thermal mass.
The amount of thermal mass must be balanced with the amount of solar glazing. The more north facing glass you have the more thermal mass you need to store the additional heat from the sun otherwise the interior could overheat, even in the middle of winter.
See “The Solar House” by Dan Chiras (Pp.102-104) for details of Glass-to-Mass Ratios.
In the Greeny Flat we have used the concrete floor as the thermal mass. We chose not to insulate under the slab in order to allow the temperature of the earth beneath the house to help moderate the interior temperatures.
We have installed a weather station that monitors, among other things, the interior and exterior temperature (circled in red).
The weather station records the data and can produce graphs like this one which perfectly demonstrates the effect of thermal mass on interior temperature. The blue line in the middle is the exterior temperature which, during these three days in July, was fluctuating between 20˚C and -2˚C. The lighter line above it shows the interior temperature which is staying fairly even between 15˚C and 25˚C. Not only is the interior slower to cool down at night, it is also slower to heat up during the day because a lot of the energy from the sun coming in is being stored in the thermal mass of the floor and the ground underneath it.