THERMAL MASS
'How does thermal mass affect the thermal comfort of Canberra homes?'
For those who live in Canberra, you will know that on hot days it can be absolutely sweltering inside. And not only that, but when it's cold, you need multiple layers of clothing to keep your body at a comfortable temperature, even when you're inside. It's not just the extreme weather we have in Canberra that causes these problems - it's also a lack of understanding about thermal mass in the design and construction of buildings.
A building has 'high thermal mass' because of its ability to absorb heat, one form of energy. Masonry materials, such as concrete and brick have a high capacity to store energy due to their density (mass). They take lengthy periods of time to absorb heat energy and long periods of time to release heat because of their high thermal mass. Materials such as timber and plasterboard have low thermal mass. In summer, it is possible that cool night air will draw out excess heat from thermal mass which then helps to keep the house cool during the daytime. In winter, heat is stored in the thermal mass during the day, and released at night to warm the home. This is shown in the diagram below.
Source: Reardon, Chris, Your Home Technical Manual - 1.7 Thermal Mass
Thermal mass is measured in 'Volumetric Heat Capacity', or kj/m³/°K. That is kilojoules per metres cubed per degree Kelvin. Different materials have vastly differing thermal mass. For example, to raise the temperature of a cubic metre of concrete by 1°K it takes approx 2060 kj of energy, whereas to raise the temperature of a cubic metre of plasterboard by 1°K it takes only 980 kj of energy. Conversely, when the temperature of a cubic metre of concrete drops by 1°K, it will emit 2060 kj of energy.
| MATERIAL | THERMAL MASS (VOLUMETRIC HEAT CAPACITY, KJ/M³/°K) |
|---|---|
| WATER | 4186 |
| CONCRETE | 2060 |
| SANDSTONE | 1800 |
| COMPRESSED EARTH BLOCKS | 1740 |
| RAMMED EARTH | 1673 |
| FC SHEET (COMPRESSED) | 1530 |
| BRICK | 1360 |
| EARTH WALL (ADOBE) | 1300 |
| TIMBER | 900 |
| AAC | 550 |
Source: Reardon, Chris, Your Home Technical Manual - 1.7 Thermal Mass
