Materials release thermal energy when room temperatures drop below that of the material. Therefore, the thermal mass limits internal temperature fluctuations by reducing the influence of external weather variations. This buffering effect can decrease reliance on intermittent heat sources such as space heaters and the sun.
The width of the thermal mass determines its influence. For example, concrete from 50-100 mm influences the daily cycle of temperature and is thus suited to temperate climates. At up to 1 metre in width, it begins to influence the weekly/monthly cycle of internal temperatures and is thus suited to hot arid climates.
Passive Thermal Mass
Thermal masses can work passively when directly exposed to internal spaces and naturally exchanging heat with the air. The effect is maximised when surface areas are increased using structures such as double tee slabs. Combined with adequate insulation, passive thermal mass can lead to significant energy savings.
To effectively reduce temperatures in summer, thermal mass is integrated with secure night-time ventilation and a diurnal range (difference between daily maximum and minimum temperatures) of at least 8°C. Pre-cooling the thermal mass during the night, removes heat absorbed the previous day and therefore recharges the mass to absorb heat the next day. If designed correctly this passive cooling approach can result in internal temperatures 2-5°C lower than the outside temperature – a margin great enough to make natural ventilation a viable strategy in the more temperate climates of Australasia.
Innovative phase-change materials embedded in ceilings or wall panels are another means of influencing passive thermal mass. These wax-like materials melt and solidify in temperatures ranging from the early- to mid-20°C, facilitating heat absorption and release. The effect is similar to that seen in traditional materials such as concrete, but utilises heat produced during phase transition. Thus, similar quantities of heat are exchanged through a thinner and lighter mass. Phase-change materials work effectively in timber and steel framed construction, allowing thermally lightweight projects to perform as heavyweight buildings would.
Active Thermal Mass
Active thermal mass systems control air flow through a voided structure. This turbulent air exchange facilitates far greater heat exchange than passive thermal mass, providing a more effective passive cooling in summer. Active air-based systems are particularly suitable for buildings with higher loads from occupants or equipment.