Thermal Mass

Concrete and other dense materials such as stone, fibre cement etc have an inherent capacity (related to their mass, thermal conductivity and exposed surface area) to absorb and store thermal energy. This quality is referred to as “thermal mass”.

Quite simply these materials will absorb thermal energy, store it, and release it when the internal room temperature drops below that of the material. This buffering effect means that the intermittent nature of heat sources such as space heaters and the sun becomes less apparent – temperature fluctuations are reduced. The thermal mass has the effect of dampening the external temperature variations.

The thickness of the thermal mass determines its influence. For Instance concrete from 50-100mm influences the daily cycle of temperature and up to 1m thick influences the weekly/monthly cycle of internal temperatures. The former thickness being more suited to temperate climates and the latter to traditional responses to hot arid climates

Passive Thermal Mass

Thermal mass can be used passively where it is merely exposed to the internal environment of the space and exchanges heat naturally between itself and the air. Thermal mass with extended surface area such as double tee slabs assists in maximising this effect.

To be effective the “ thermal mass” needs to be exposed directly to the internal, heat gathering, spaces and not buried behind wall linings. When combined with adequate insulation significant energy savings can be achieved. To be fully effective in limiting summertime temperature rise thermal mass needs to be combined with a secure means of night-time ventilation and a diurnal range (difference between daily maximum and minimum temperatures) of at least 8°C. Pre-cooling of the thermal mass by night time air removes the heat absorbed during the day and ‘recharges the thermal mass to absorb more heat the next day. If designed correctly this passive cooling approach can result in the internal temperature being 2-5°C lower than the outside temperature which is often enough to make a natural ventilation strategy viable in the more temperate climates of Australasia.

The adjacent graph for one of our projects, the MSCS building in Christchurch demonstrates how effective thermal mass is in evening out temperature variations in the naturally ventilated academic offices in the MSCS building. The graph is taken from an RAIA case study on the building, ( notes/CAS56.pdf), by Professor George Baird at Victoria University, Wellington.

New phase change materials which encapsulate a wax like material into a ceiling or wall panel provide another means of providing passive thermal mass. The wax core melts and solidifies in a range in the early to mid twenties centigrade enabling heat to be absorbed and released in a similar way to the sensible heat transfer of traditional thermal mass materials such as concrete but utilising the latent heat of the phase change. This enables a similar quantity of heat to be exchanged in a thinner and lighter way making it appropriate for timber and steel framed construction. In effect converting a thermally lightweight construction into a heavyweight one. We proposed this approach for our competition scheme for the all timber construction NMIT building.

Active Thermal Mass

Active thermal mass involves forcing air over or through a high mass voided structure. This turbulent air exchange promotes far greater heat exchange than passive thermal mass and allows it to provide more passive cooling in summer. This makes it more suitable for buildings with higher load characteristics from occupants or equipment. Examples we have used this approach include the precast sinusoidal floor structure at the MSCS building and precast shell beams at the Population Heath Complex and the Waitakere Central Building.