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How Thermal Mass Works

The thermal capacity of the building's elements delays the heat transfer to the interior of the building, by soaking up excessive heat for several hours. During the night, when the external temperature is lower, the stored heat is slowly expelled to the environment by radiation and by convection.

  1. Heat is radiated through the surface of the mass by a warmer object (such as sun, lights, people, or equipment).
  2. Heat is conducted from the warmer surface of the mass to the cooler interior of the mass, effectively "storing" heat in the mass.
  3. When the mass surface becomes warmer than other objects surrounding it, the mass radiates heat to these objects (meaning the mass radiates heat back into the house).
  4. Heat from the warmer interior of the mass is conducted to the surface of the mass as the mass cools (a reversal of step 2).


The most effective construction materials are those with the highest volumetric heat capacity. In general, dense materials will generally have a higher thermal mass than less dense products. For example, dense concrete blockwork, rammed earth and mud bricks have a high effective thermal mass when compared to lightweight blockwork or wood.

For thermal mass to be effective there must be minimal thermal resistance between the occupied space and the mass of the structure. The temperature fluctuations within the building fabric are greatest at the surfaces. Relatively thin layers of plaster can have a significant effect on the thermal mass by providing thermal resistance.

The Seasonal Effects of Thermal Mass


In summer, thermal mass absorbs heat that enters the building. In hot weather, thermal mass has a lower initial temperature than the surrounding air and acts as a heat sink. By absorbing heat from the atmosphere the internal air temperature is lowered during the day, with the result that comfort is improved without the need for supplementary cooling.

At night the heat is slowly released to passing cool breezes (natural ventilation), or extracted by exhaust fans, or is released back into the room itself.


In winter, thermal mass in the floor or walls absorbs radiant heat from the sun through south, east and west-facing windows. During the night, the heat is gradually released back into the room as the air temperature drops. This maintains a comfortable temperature for some time, reducing the need for supplementary heating during the early evening.

The most difficult period in winter is the early morning. The heat released during the night has dissipated, temperatures have dropped and the sun has yet to begin the heating process. During this time it will probably be necessary to use supplementary heating to warm the thermal mass before the air temperature rises.

Locating Thermal Mass

  • Thermal mass is most effective where exposed to direct sun radiation.
  • Where not exposed to direct radiation, thermal mass relies on efficient convection.
  • Comfort is improved if the mass is distributed evenly within a room.
  • Thermal mass should be insulated from external temperatures for maximum effectiveness.
  • Materials that make for effective thermal mass usually perform badly as insulators.
  • The most important location for thermal mass is in south-facing rooms. To heat thermal mass effectively in winter, it should be optimised for exposure to direct winter sun.
  • As the area of south-facing window increases, the more thermal mass is required to maintain a stable temperature.
  • Thermal mass located within north-facing rooms is relatively un-important. It is frequently argued that thermal mass should be avoided altogether in bedrooms, so reducing an associated nocturnal rise in temperature.
  • Summer conditions can lead to overheating to eastern and western facades. Consideration should be paid to locating thermal mass in these locations.
  • Locate additional thermal mass near the centre of the building, particularly if a heater is positioned here.

Contact Details

Passive Building Structures,
Dernawilt Rd, Roslea,
Co. Fermanagh BT92 7FH,
Northern Ireland.

Call: 03301332511

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