Understanding Thermal Mass & Insulation

How to avoid a long hot summer inside

It was a typically hot summer day on 11 January, 2013 and the thermometer was rising fast at CSR House in Schofields in Sydney’s West. While the ambient temperature peaked at 40°C, the external brick wall of CSR House recorded 64°C in direct sunlight at 5.30pm.

However, the internal brick wall only reached a peak temperature of 29°C. This and other tests conducted at the height of summer at CSR House perfectly demonstrated the role of insulated thermal mass in maintaining comfortable temperatures in buildings throughout summer.

Conversely, during winter the internal skin of the cavity brick wall at CSR House maintained a minimum temperature of 17°C, while the outside ambient temperature dropped to just above freezing (2°C). The external brick skin recorded a temperature of 5°C.


The use of thermal mass with insulation in a walling system is useful to slow the transfer of extreme heat and cold by acting as a temperature sink that slowly releases the stored temperature over an extended period. The net result is that the temperature within the house remains stable within a comfortable range, which reduces the occupant’s desire to turn on the air-conditioner.

Thermal mass is the capacity of a material to absorb heat energy. A lot of heat energy is required to change the temperature of high density materials like concrete, bricks and tiles. They are therefore said to have high thermal mass. Lightweight materials such as timber and composite cladding have low thermal mass.

Appropriate use of thermal mass throughout a home can make a really big difference to comfort while keeping a lid on heating and cooling bills. The correct use of thermal mass moderates internal temperatures by averaging day/night (diurnal) extremes, in turn increasing comfort and reducing energy costs.

Poor use of thermal mass can exacerbate the worst extremes of the climate and can be a huge energy and comfort liability. It can radiate heat all night during a summer heatwave, or absorb all the heat on a winter night.

These findings are supported by thermal performance research conducted by Thinkbrick Australia in conjunction with the Faculty of Engineering and the Built Environment at the University of Newcastle. This research found that designing and constructing an energy efficient house has the potential to substantially reduce the amount of energy consumed by incorporating thermal mass into the wall structure.

The research showed that insulated cavity brick walls ensure that all components of the construction work together to reduce the transfer of heat, whereas brick veneer and lightweight cladding walls rely mostly upon the insulation. To compensate for a lack of thermal mass, walling systems incorporating lighter weight cladding materials generally require more insulation to achieve similar comfort levels.

With larger cavities there is much better potential to use more insulation to control heat energy and to ensure suitable moisture drainage planes and drying cavities for more extreme climates.

In tests at CSR House an unusually large cavity* of 200mm was used for R6.0 insulation to be installed within a double brick wall, so the benefits of thermal mass were maximised. This decisive use of insulation to thermally break or decouple the heat stored within the external skin from reaching the all-important internal skin certainly helps to regulate the temperature inside CSR House.

If the temperature of the internal brick skin remains stable then thermal comfort is more easily achieved. When sufficient insulation is installed between the internal and external skins, the conditions found on either side of the insulation become more and more independent of each other.

* CSR Building knowledge is not suggesting houses should be built with a 200mm cavity; that would be too expensive. The traditional practice of leaving only a 50mm gap is insufficient to achieve excellent thermal performance in very hot and cold conditions. Designers need to consider larger cavities to enable increased insulation R values for better performing cavity brick walls.

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