What is thermal mass?
Scientifically, thermal mass is the same as thermal capacitance or heat capacity, the ability of a body to store thermal energy. Thermal mass is typically described by the symbol C and measured in J/C° or J/K. Any solid, liquid, or gas that has mass has some thermal mass. However, when talking about thermal mass in building design it is referred to as the mass of a building, which is able to store heat.
How thermal mass works
Thermal mass functions as a thermal battery that can help to even out temperature fluctuations. Thermal mass is especially useful in climates with big temperature differences between day and night. At summer time it absorbs heat during the day and releases it by night to the cooler environment, keeping the house comfortable. In winter the same thermal mass can store heat from the sun and indoor heating and release it at night, helping the building to stay warm. A high thermal mass is not a substitute for insulation. Thermal mass stores and re-releases heat while insulation stops heat flow into or out of a building.
Ideal materials as thermal mass
For materials to provide a useful level of thermal mass they need a combination of three basic properties:
- High specific heat capacity – maximizes the heat in every kg
- High density – the heavier the material, the more heat it can store
- Appropriate thermal conductivity – so the rate heat flows in and out of the material is in step with the daily heating and cooling cycle of the building
Heat capacity is a measure of how much heat that is needed to change the temperature of matter. The origin of heat capacity is in thermally induced molecular movements. The molecules that have more freedom, greater ability to move, can store more heat and have higher specific heat capacity. High specific heat capacity is how much energy that is needed to change the temperature of a unit mass of a substance with one degree. Volumetric heat capacity (VHC), is occasionally used in engineering and is the high specific heat capacity of a given volume. Specific heat can be converted into VHC by multiplying the specific heat with the density of the material.
Conduction is the process in which heat moves through a material, or from one material to another if in contact. Thermal conductivity measures the rate heat flows through a material when a temperature difference exists between its surfaces. The thermal conductivity varies with density, porosity, moisture content of the material and the absolute temperature. If the thickness of the material is increased, the conductance decreases proportionately.
Different material as thermal mass
Heavyweight constructions made from brick, stone and concrete have the three earlier mentioned properties that provide a useful thermal mass. They combine high storage capacity, high density and moderate thermal conductivity. Other material such as wood, have high heat capacity but low thermal conductivity, which limits the heat that is being absorbed and released. Steel can also store heat but has very high thermal conductivity, which means that heat is absorbed and released too fast to be in sync with the natural heat flow of the building.
Night time cooling
During the hot summer months, the building needs to be cooled during the night by ventilation to remove heat that has been absorbed during the day to avoid overheating. Night time cooling can be achieved by opening up pathways for wind ventilation and stack ventilation at night time to cool down the thermal mass in the building. In the morning the vents are closed off to prevent warm air from entering the building. The openings can be opened manually or automatically. There are some limitations to using night time cooling such as usability, security and reduced indoor air quality. All which are depending on the surrounding of the building and the openings.
Using the ground as thermal mass
Thermal mass can be stored in 3 types
- Sensible – storage based on heat capacity
- Latent – storage based on phase change
- Chemical – storage based on chemical processes
Sensible heat storage is when the temperature is gradually changing as heat is added or removed from a mass. Almost all buildings can apply sensible heat storage. The only condition is that the temperature varies. There is no exchange of heat between the thermal mass and the indoor environment in a building with constant temperature. A modern use is storage of warm water, by using either water accumulators or underground natural buffers. In latent heat storage the storage material changes its phase, usually from liquid to solid and vice versa. It requires a large amount of energy to for example melt ice to water, and can because of this be used as cooling. Latent heat storage requires a lower mass and volume to same amount of heat as in sensible storage. A modern phase changing material is pellets of paraffin manufactured with different melting points adapted for different purposes. Chemical heat storage consists of materials that can undergo reversible chemical reactions, which is energy consuming in one direction and energy contributing in the other. Sodium acetate heating pad is an example of chemical heat storage.
References and links:
Karlsson, J. (2012) “Possibilities of using thermal mass in building to save energy, cut power consumption peaks and increase the thermal comfort”, Lund
Fathy, H. (1986)“Natural Energy and Vernacular Architecture”, Chicago