Concrete Sector Sustainability Working Group (CSSWG)

Thermal mass basically describes the ability of construction materials to absorb, store and release heat; a useful property which helps regulate the temperature in buildings. Heavyweight materials such as concrete provide a high level of thermal mass, and this is often measured in terms of something called 'admittance' which has units of W/m² K.  An example of a low thermal mass construction is a timber frame wall, which has an admittance value of around 0.85 W/m² K. An example of a high thermal mass construction is a traditional brick and block wall with a plaster finish which has an admittance of around 6 W/m² K. In other words, a heavyweight wall can have around seven times more thermal mass than a lightweight wall. This should not be confused with insulation and U values which are entirely independent of thermal mass and can be exactly the same for heavy and lightweight construction.

Alongside effective ventilation and solar shading, the use of thermal mass is becoming an increasingly important passive design feature in buildings, driven by the need for adaptive design measures aimed at minimising the risk of overheating problems linked to climate change. This is achieved through the ability of thermal mass, in heavyweight floors and walls, to absorb internal heat gains during hot weather, helping stabilise the internal temperature. In buildings such as offices, the peak internal temperature is also delayed by around six hours, which will typically occur in the late afternoon, or evening after the occupants have left. At this point, heat gains from office equipment, lighting, occupants and the sun are greatly diminished and the building fabric stops absorbing heat. As the evening progresses, the external air temperature drops, making night ventilation an effective means of removing accumulated heat from the building and lowering its temperature in readiness for the following day. To make this heating and cooling cycle effective, it is essential that heavyweight concrete elements, such as floor slabs, are thermally exposed (e.g. an exposed concrete soffit) so heat can move freely between the internal environment and the concrete.

 

slabs (Architect: Feilden Clegg Bradley).

 

The ability of thermal mass to absorb heat also results in a lower cooling load in air conditioned buildings, helping reduce associated CO₂ emissions and energy use. In many cases, air conditioning can be avoided altogether through the use of thermal mass and effective ventilation; a combination that has been used very successfully in many commercial and public sector buildings. Information on thermal mass in non residential buildings can be found in The Concrete Centre Guide: 'Thermal Mass', which is free to download from The Concrete Centre website. A more detailed design guide on the subject entitled: 'Utilisation of Thermal Mass in Non-Residential Buildings' can be purchased from the Concrete Bookshop.

 

The ability to avoid the need for air conditioning is also becoming increasingly relevant for dwellings, with the prediction that many homes are going to suffer from excessive overheating as the 21^st century progresses ¹; particularly lightweight dwellings where the problem is generally more acute. Met Office figures for 2006 show the average air temperature continues to indicate a warming climate, both around the world and especially in the UK. In central England, the highest average yearly temperature was recorded since records began, and perhaps more significantly, extended hot periods have also broken previous records.

 

The ability of thermal mass to avoid or reduce overheating problems is relatively well known, perhaps less well known is its ability to save energy during the heating season, a technique often referred to as passive solar design (PSD). Passive solar energy is nothing new; about 14% of space heating in an ordinary UK home comes from solar energy through walls and windows ². However, through PSD, this can be significantly increased through larger south facing windows combined with a medium to high level of thermal mass to absorb and store heat on winter days. As night falls and the temperature drops, the stored heat is slowly released into the building, helping reduce the load on the boiler or other heating system.

 

This process is effectively the same as that which occurs on summer nights, the only difference being that during the winter the stored heat is beneficial, so windows and openings are kept shut to minimise heat loss. Shutters and blinds used to prevent overheating in the summer can also help insulated windows in the winter, increasing the effectiveness of PSD.

 

 

It can be seen that where PSD is used, medium and heavyweight buildings (particularly dwellings) can exploit their inherent thermal mass on a year round basis. Recent improvements to standards of insulation and airtightness in dwellings have also made the use of PSD a much more effective means of saving heating fuel. It is also worth noting that the whole life CO₂ savings provided by the appropriate use of thermal mass can far outweigh any increase in embodied impacts resulting from the use of concrete ³.  

 

Information on thermal mass in housing can be found in The Concrete Centre guide: 'Thermal Mass for Housing' which is free to download from The Concrete Centre website.