Concrete @ your Fingertips

Thermal mass of concrete

When we touch concrete we feel it as a ‘cold’ material. However concrete is also used in electrical night storage heaters. Why? The answer is ´Because of its high thermal mass´.

The thermal mass, or heat capacity, of a material plays an important part in designing an efficient and comfortable structure. It is a measure of how much heat a material can hold. Water has a heat capacity of 4.2 kJ/kgC whereas many building materials are in the range 0.8 to 1.3 kJ/kgC. This property is significant for heavy, high thermal mass materials where the heat capacity is calculated from the volume and the specific heat of the material. The ratio of the surface area exposed to the volume affects the rate at which the heat is absorbed and released.

Concrete can be used to absorb heat to keep the interior of a building cool throughout the day, but overnight natural ventilation can be used to cool the concrete down and warm the room space. The pattern is repeated each 24 hours. Approximately 50% of UK CO2 emissions come from heating, lightig and cooling buildings. Clearly reducing the need for air-conditioning or space heating can have a significant impact on this figure. (See also Environmental aspects/Structure and associated sub-entries.)

Ideally modern buildings should be constructed in such a manner as to minimise temperature build up in the room space during warm weather and yet prevent the loss of this excess heat in cold periods. To achieve this, a combination of insulation to exterior walls is required for colder weather and a high thermal mass to act as a heat sink for hot weather. Concrete has a high thermal mass with properties similar to brick and stone. It is possible to absorb heat from the atmosphere in warm weather and release it during cooler periods, e.g. overnight. This is known as the ‘thermal flywheel’ effect. In a passive concrete design the cooling capacity of concrete can be up to 25W/m2 and with an active system, e.g. by ducting of air through a concrete slab, up to 40W/m2 can be absorbed.

As well as being able to act as a passive air conditioning system for buildings, concrete and cementitious-based products have good sound insulation and deadening properties

For further information see The thermal mass of concrete, published in CONCRETE in October 2003, p52 and The changing climate for thermal mass, published in CONCRETE in June 2005, pp. 7–9. Copies are available as free downloads from the Members Area of the Concrete Society web site.

Acknowledgement: UKQAA