Thermal mass describes how materials such as masonry and concrete are able to absorb heat during hot conditions and then release it when it’s cooler, typically at night. This provides inertia against temperature fluctuations within a building and helps lower the risk of overheating (see overheating). Although less common in the UK, thermal mass can also be used in the design of very low energy homes to capture solar gains during the winter months and recycle internal heat from people and appliances, helping reduce heating bills.
The performance benefits that masonry and other materials with thermal mass can offer on a year-round basis are recognised to some extent within the SAP tool used to calculate the energy/carbon performance of homes and demonstrate Part L compliance. Masonry offers a combination of three basic characteristics which are required to provide a useful level of thermal mass:
1. A relatively high specific heat capacity; so the heat squeezed into every kg is maximised.
2. A relatively high density; the heavier the masonry, the more heat it can store by volume.
3. Moderate thermal conductivity – so the rate heat flows in and out of the masonry is roughly in step with the daily heating and cooling cycle of the building.