A critical part of the CHEESE Heatview® surveyor’s training is understanding a set of concepts in Physics and how they apply to building design and building materials. One such concept is thermal mass.
To be able to calculate the thermal mass of any given material, we need to measure its specific heat capacity. Therefore let’s wrestle with the meaning of specific heat capacity first!
Weight > Volume
However, when we refer to the heat capacity of different materials in the building industry, we talk about volumetric heat capacity (VHC). Volumetric heat capacity takes the density of different materials into account. Volumetric heat capacity is also known as thermal mass.
Still reading?
Thermal mass is the ability of a material to absorb, store and release heat. The higher the VHC, the higher the thermal mass. We calculate the thermal mass of a material by multiplying its specific heat capacity with its density.
Equation: Specific Heat Capacity multiplied by the Density of the material
Thermal mass of various materials:
| Material | Density (kg/m3) | Specific heat capacity (kJ/kg.C) | Volumetric heat capacity (kJ/m3.C) |
| Water | 1000 | 4.186 | 4186 |
| Concrete | 2240 | 0.920 | 2060 |
| Stone (sandstone) | 2000 | 0.900 | 1800 |
| Compressed earth blocks | 2080 | 0.837 | 1740 |
| Rammed earth | 2000 | 0.837 | 1673 |
| Fibre cement sheet (compressed) | 1700 | 0.900 | 1530 |
| Brick | 1700 | 0.920 | 1360 |
| Earth wall (adobe) | 1550 | 0.837 | 1300 |
| Autoclaved aerated concrete (AAC) | 500 | 1.100 | 550 |
Q1:
“Does air have thermal mass?“
Read on!
The outside and the inside temperatures are in constant flux. Every wall construction is a piece of technology. Its primary role is to keep habitants dry, warm/cool and protected against elements. The thermal mass of a wall construction is responsible for moderating daily temperature fluctuations.
A light-timber framed building looses more heat during the night and heats up quicker during the day: its inside temperatures fluctuate relatively high because of its low thermal mass. Whereas the inside temperatures in a heavy building with layers of external insulation evens out the extremes of a day-night cycle much more. Constructions with high thermal mass keep the inside temperatures “steady”, they don’t loose heat quickly, neither do they absorb heat quickly! Heating up a room in a low thermal mass environment requires less energy because the walls warm up quicker and the air in the room will feel warmer sooner but they don’t store heat as well as constructions with high thermal mass. The air feels warmer sooner but it looses its heat quicker too and spaces need heating more frequently. Insulation levels and air tightness will also influence how long the captured heat is held within the home. Insulation and thermal mass are two different concepts in building design although often conflated. They perform different roles: insulation reduces heat transfer between the outside environment and the building envelope, thermal mass moderates internal temperatures by storing and releasing heat energy overtime (thermal lag).
Thermal mass is usually a consideration when building a home, but some changes can be retrofitted.
Looking at the bar chart listing the specific heat capacity of various materials, you can see that water has a very high thermal mass, the highest of any common material on Earth.
Q2:
Could we utilise the high thermal mass of water in the construction industry? Walls made out of water?
The optimal thermal mass of a building depends on the climate it is situated. The difference between day and night outdoor temperatures vary at different parts of the Earth. Thermal mass is most appropriate in climates with a larger (“diurnal”) temperature range.
A1:
Volumetric Heat Capacity of Air: 0.0012 (MJ/m3K)
For comparison:
Concrete: 2.086 (MJ/m3K)
Water: 4.18 (MJ/m3K)
“A common misconception is that only concrete or earth soil has thermal mass; even air has thermal mass (although very little).”
A2:
References:
- yourhome.gov.au/passive-design/thermal-mass
- energyeducation.ca/encyclopedia
- theyearofmud.com/insulation-thermal-mass-differences
- theengineeringmindset.com/specific-heat-capacity-of-materials
Additional reading:
- How buildings work, Huw M. A. Evans, RIBA Publishing, 2016
Written & Visualisation by Dori Jo