My house at Manilla, NSW, is in a climate with temperatures that are extreme, but comfortable on the average. To reduce extreme temperatures indoors, the house contains more than a hundred tonnes of thermal mass within a shell of insulation.
The “thermal mass” is the materials, such as bricks, stones, concrete, earth or water, that have high thermal capacity (See Notes below): they take in and give out a lot of heat.
Many people, who can see that having thermal mass inside a house will help to keep it warm in winter, think that the thermal mass will make it hard to keep the house cool in summer. They see many brick and brick-veneer houses in which thermal mass is exposed to the intense heat of the summer sun. In that case, thermal mass material does no good.
In this graph, I have used my last twelve months of temperature data to show the benefit of well-insulated thermal mass in summer as well as in winter.
Outdoor temperature in this year went as low as minus 4.0° Celsius and as high as plus 43.7°: a range of 47.7°.
The total effect of orientation, glazing, insulation, shading, ventilation and thermal mass was to reduce the range of room temperature to 16.4°, and to make it agree closely with the adaptive comfort zone. At Manilla, the comfort zone goes down to 17.3° in winter, and goes up to 29,3° in summer. Room temperature went down only to 15.0°, and went up only to 31.4°. Not many hours in the year were beyond the comfort zone limits.
I show the subsoil temperature that I observed at 750 mm depth below the surface of the soil in the garden, and at the same depth in the footings below the floor slab. Clearly, the house protects the mass below the slab from much of the temperature variation that occurs below the garden. While the mean temperature is almost the same, the temperature range is reduced from 13.9° to 5.8°. I have limited any heat flow between the subsoil under the garden and that under the house by polystyrene perimeter insulation in the footings.
By contrast, heat is conducted freely up and down between the under-slab heat bank and the house: the house moderates the heat bank temperature and the heat bank moderates the house temperature. The fact that the subsoil at 750 mm below the floor slab did not get warmer than 23.3° prevented the surface of the slab from getting warmer than 28.6°, nearly 3° cooler than the maximum room temperature.
Definitions (see Wikipedia):
Thermal capacity: the ratio of the heat added to (or subtracted from) an object to the resulting temperature change. Units: Joules per degree kelvin (Note: the mass of the object is included.)
Specific heat capacity (or specific heat): Thermal capacity per unit mass.
Thermal mass materials: In building, these are materials with high density, mainly above 1000 kg/m^3, and high specific heat, mainly above 800 joules per kilogram degree kelvin.