Thermal Mass
Thermal Mass, what is it and why is it important?
Physical definition
Thermal mass is also known as volumetric heat capacity (VHC) and is the product of the specific heat capacity (SHC) and the density (p) of the object.
VHC is the energy required to increase a unit volume of a material by a unit temperature (1 m3 by 1 C). It can be used as a mass descriptor, such that materials that have a “heavier” thermal mass require more energy to change the temperature of
Thermal lag is the rate at which energy is released from a material.
- VHC: Volumetric Heat Capacity a.k.a. Thermal Mass of a material, (J/m3/degC).
- SHC: Specific heat capacity of a material, (J/kg/degC). SHC is the energy required to increase a unit mass of a material by a unit temperature (1kg by 1C).
- p: density of a material, (kg/m3)
Back to what it means for you home
Even though thermal mass is a physical property of all materials, thermal mass can be used to refer to a material if this energy-storing property is its main function. Thermal mass can be added to spaces or houses to assist in the thermal performance of the space. These are often heavyweight objects with high thermal mass and long lag times, essentially acting as passive heaters.
Examples of materials with high thermal mass include brick, concrete and stone. You may notice the temperature lag over the course of a day as brick can remain cool into the late morning and stay warm into the evening.
Materials with low thermal mass can include timber and other lightweight materials. Often the temperature does not vary much from the air around it, not in a way that is often noticed.
Thermal mass is a property of all materials, and your home will always be influenced by this. You can, however, optimise the thermal mass according to your climate. If the temperature varies throughout the day, then you may benefit from increasing the thermal mass. Warmer climates where the temperature does not vary significantly (North Queensland and the Top End) benefit more from lighter thermal mass. Often timber is used for well-ventilated housing in these areas. If, however, air-conditioning is intended to be used often in the space, then a higher thermal mass is recommended. This reduced the loss of cool air through the walls and floor when the aircon is on indoors.
Thermal mass is helpful to understand if you are trying to determine what materials to construct your home of, and can have a significant influence on the passive thermal performance of your home.