Thermal Mass

I was sent a very interesting thesis entitled ‘A Comparative Study of the Effects of Thermal Mass in New Dwellings in Scotland’ by Janice Foster. The paper researches how thermal mass affects overheating in buildings.

Most people would intuitively say that thermal mass would prevent buildings from overheating by absorbing the excess heat from either solar gains or internal gains and in some cases this is true. However, as with all things related to buildings and people, it is not that simple.

First and foremost there needs to be adequate night time cooling to dissipate the heat stored from the day. If this is not present then the thermal mass will exacerbate the overheating by radiating heat in addition to the other gains. Secondly, to actually work the mass need to be thermally coupled with the interior of the building. Simply, this means that the mass is in direct contact with the interior of the building and not hidden under carpets, behind service voids or in the case of ICF, not behind a layer of insulation.

The thesis also discusses how thermal mass, used correctly, is more useful in the South and East of the country where there is more sunshine and the average temperature is higher. High mass buildings typically have an internal temperature around 2 degrees lower than lightweight buildings and so as the climate changes developers and homebuyers should be looking for heavier-weight constructions.

There were two surprising points within the document, both around the cooler end of the year. One was that high mass buildings can reduce heating requirements by a staggering 20% by simply reducing the fluctuations in internal temperatures and storing heat gains from the day. The second was the gap between real energy consumption data and that calculated by modelling software such as IES and SAP. On one building SAP calculated the energy consumption to be 20kWh/m2, IES was 35kWh/m2 whereas the actual consumption of the building was 89 kWh/m2. Such staggeringly inaccurate assessments, especially by a tool which is a mandatory requirement of building control, beggars belief. How on earth can we pretend to be able to model buildings and make informed decisions on how to reduce heat loss when the output of such programs is so monumentally wrong?

The final point to make is about orientation and the difference it makes to heating and cooling requirements. South really is by far the best orientation from an overheating and a heating requirement point of view. To see a copy of the thesis contact Janice Foster at

Chris Brookman
Chris Brookman