What is Thermal Mass?
11 Mar 2019
Thermal mass is a concept in building design that describes how the mass of the building provides inertia against internal temperature fluctuations. This is typically achieved through its ability to absorb unwanted heat during the day and then release it at night with the help of ventilation from cool night air. For a material to provide a useful level of thermal mass, a combination of three basic characteristics is required:
- A high specific heat capacity; so the heat squeezed into every kilogramme is maximised.
- A high density; the heavier the material, the more heat it can store by volume.
- Moderate thermal conductivity – so the rate heat flows in and out of the material is roughly in step with the daily heating and cooling cycle of the building.
Heavyweight construction materials such as masonry and concrete have these characteristics. They combine a high storage capacity with moderate thermal conductivity.
This means that heat moves between the material’s surface and its interior at a rate that roughly matches the building’s daily heating and cooling cycle. Some materials, like wood, have a high heat capacity, but their thermal conductivity is relatively low, limiting the rate at which heat can be absorbed during the day and released at night. Steel can a store a lot of heat, but conducts it too rapidly to be particularly useful, plus comparatively little is used in buildings.
However, a modest amount of thermal mass may still be provided if concrete floors are used in steel frame construction, although these are usually limited to a depth of only 100mm and are usually covered by a false ceiling, limiting their ability to absorb and release heat.
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