Thermal mass is the property of a material that enables it to absorb, store, and emit heat energy. It is also kwon as thermal capacitance or heat capacity. Thermal Mass is a commonly used concept in passive building design.

Materials with high density such as concrete, bricks etc require a lot of heat energy to increase their temperature. They are therefore said to have high thermal mass. Lightweight materials such as timber have low thermal mass.

Thermal mass acts as a heat sink. It absorbs heat from the environment and the interior space, keeping the house comfortable during the summer. The same thermal mass can release the heat at night, keeping the home warm during the winter.

When used well and combined with passive solar design, thermal mass can make a big difference in terms of comfort and energy use of a building by averaging day/night (diurnal) extremes. But Poor use of thermal mass can exacerbate the worst extremes of the climate and can be a huge energy and comfort liability.

Units and Measures

In the case of a homogeneous material with kwon physical property, the thermal mass is the mass of material times the specific heat capacity of that material. For composite materials, the sum of heat capacities of the individual materials may be used for calculation, or in some cases the whole body of the composite material may be considered as one material.

In building science, volumetric heat capacity is generally in use instead of specific heat capacity. Volumetric heat capacity depends on the volume of the material, while the specific heat is based on the mass of the material. If given a specific heat value of a substance, one can convert it to the volumetric heat capacity by multiplying the specific heat by the density of the substance.

Following are the units for measuring thermal mass:

  • SI Units: Joule per cubic meter kelvin or J/(m³•K)
  • Imperial Units: British thermal unit per cubic feet degree fahrenheit or BTU/(ft³•F°)

Example Values

Following are the thermal mass (volumetric heat capacity, KJ/m³.k) of some commonly used building materials:

  • Water - 4186
  • Concrete - 2060
  • Sandstone - 1800
  • Compressed earth blocks - 1740
  • Rammed earth - 1673
  • FC sheet (compressed) - 1530
  • Brick - 1360
  • Earth wall (adobe) - 1300

Useful References

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