Thermal mass has the ability to flatten out daily temperature fluctuations
Thermal mass is the ability of a material to absorb and store heat energy. A lot of heat energy is required to change the temperature of high density materials like concrete, bricks and tiles. They are therefore said to have high thermal mass. Lightweight materials such as timber have low thermal mass. Appropriate use of thermal mass throughout your home can make a big difference to comfort and heating and cooling bills.
Thermal mass works in the colder months by absorbing heat energy from the sun during the day and slowly releasing this heat during the evening and night thus “flattening out” the day and night temperature fluctuations.
Ideal high density thermal mass materials include concrete, bricks, rammed earth, stone, tiles and water.
Thermal mass is also beneficial in summer. In summer, the interior of the house should be shaded. The thermal mass will stay cool and absorb any unwanted heat by conduction from the air. At night, this mass can be cooled by opening the house to the cool night air ready for the following day.
Thermal mass is not a substitute for insulation. Thermal mass stores and re-releases heat; insulation stops heat flowing into or out of the building. A high thermal mass material is not generally a good thermal insulator.
Thermal mass is particularly beneficial where there is a big difference between day and night outdoor temperatures. As a rule of thumb, diurnal ranges of less than 6°C are insufficient; 7°−10°C can be useful depending on climate; where they exceed 10°C, high thermal mass construction is desirable.
In cool or cold climates where supplementary heating is often used, houses benefit from high mass construction regardless of diurnal range (e.g. Hobart 8.5°C). In tropical climates with diurnal ranges of 7°−8°C (e.g. Cairns 8.2°C) high mass construction can cause thermal discomfort unless carefully designed, well shaded and insulated.
Where to locate thermal mass
To determine the best location for thermal mass you need to know if your greatest energy consumption is the result of summer cooling or winter heating.
Heating: Thermal mass should be in areas that receive direct sunlight or radiant heat from heaters.
Heating and cooling: Thermal mass should be located inside the building on the ground floor for ideal summer and winter efficiency. The floor is usually the most economical place to locate heavy materials, such as concrete.
Thermal mass should be in north-facing rooms with good solar access, exposure to cooling night breezes in summer, and additional sources of heating or cooling (heaters or evaporative coolers).
Additional thermal mass can be located near the centre of the building, particularly if a heater or cooler is positioned there. Feature rammed earth walls, brick, stone, slabs, or a water features can be used.
Cooling: Thermal mass should be protected from summer sun with shading and insulation if required. Allow cool night breezes and air currents to pass over the thermal mass, drawing out all the stored energy.
Lightweight construction such as timber frame with weatherboard or sheet cladding respond to temperature changes quickly. Shaded lightweight construction is better suited to very hot and humid tropical areas where it is hot day and night, for sleeping spaces and evening living rooms.