Thermal mass

Thermal mass, in the most general sense, describes the ability of any material to store heat.

For a material to provide a useful level of thermal mass, a combination of three basic properties is required:

• High specific heat capacity - to maximise the heat that can be stored per kg of material.
• High density - to maximise the overall weight of the material used.
• Moderate thermal conductivity - so that heat conduction is roughly in synchronisation with the diurnal heat flow in and out of the building.

Heavyweight construction materials such as brick, stone and concrete all have these properties.

They combine a high storage capacity with moderate thermal conductivity. This means that heat transfers between the material's surface and the interior at a rate that matches the daily heating and cooling cycle of buildings. Some materials, like wood, have a high heat capacity, meaning that their thermal conductivity is low, which limits the rate at which heat is absorbed. This cominbation, which results in low thermal mass, can be useful in other ways. Steel can also store a lot of heat, but in contrast to wood, steel possesses a very high rate of thermal conductivity, which means heat is absorbed and released too quickly to create the lag effect required for the diurnal temperature cycle in buildings.

Concrete and masonry steadily absorbs heat that comes into contact with its surface, conducting it inwardly, and storing it until the surface is exposed to cooler conditions and its temperature begins to drop. When this occurs, heat will begin to migrate back to the cooler surface and be released. In this way, heat moves in a wave-like motion alternately being absorbed and released in response to the variation in day and night-time conditions.

The ability to absorb and release heat in this way enables buildings with thermal mass to respond naturally to changing weather conditions, helping to stabilise the internal temperature and provide a largely self-regulating environment. When used appropriately, this stabilising effect helps to prevent overheating problems during the summer and reduces the need for mechanical cooling. Similarly, the ability to absorb heat can help reduce fuel usage during the heating season by capturing and later releasing solar gains and heat from internal appliances.

Benefits of thermal mass

Exploiting thermal mass on a year-round basis is not difficult, but does require consideration at the outset of the design process when requirements for the building form, fabric and orientation are being established. Providing this is done sympathetically, a more passive approach to design can realise benefits which include:

• Enhanced energy efficiency and carbon savings over the life of the building.
• Improved daylighting.
• Improved ventilation and air quality.
• Optimal decrement delay (time lag) and decrement factor (heat flow) for reducing heat gains in summer.
• Good summertime comfort and a reduced risk of overheating.
• A measure of future proofing against the effects of a warming climate.
• Reduction in the need for more expensive low and zero carbon technologies to meet CO2 targets.
• Enhanced property resale value.

Concrete proof

Oriel High School, Crawley features an in-situ concrete frame with precast concrete wall panels, insulated and rendered externally. The underside of the floor slabs at both levels, which are exposed concrete cast in situ, provides thermal mass that can be cooled overnight via opening windows to help to moderate internal temperatures when daytime external temperatures are high.