Well designed houses use passive design elements to reduce not only the burden on the occupant’s wallet, but also the burden on the environment. Stephanie McDonald reports on one way designers can lessen both burdens with bricks.
Thermal mass most commonly takes the form of clay bricks or concrete slabs in homes. Using thermal mass in walls helps delay the time it takes for heat to pass through a wall, called thermal lag. This means on hot days in summer, the house takes longer to heat up inside because the heat is stored in the thermal mass for a period of time.
Using bricks for thermal mass can help heat to be absorbed deeper into the material and therefore distributed over a larger volume. “Thermal mass acts to moderate temperature fluctuations naturally by having the ability to absorb and store heat energy. More specifically, thermal mass describes the ability of a material to retain heat energy when subjected to a temperature differential. The combination of thermal capacitance and heat conduction make brickwork an extremely effective ‘thermal battery’,” says Cathy Inglis, technical advisor at Think Brick Australia.
All of this results in less heating and cooling requirements and helps temperatures to be moderated naturally. “In comparison, a lightweight, well insulated home with no mass depends on artificial heating and cooling for comfort. Insulation will reduce the amount of heat through a wall, but it cannot delay it,” Inglis says. “So without thermal mass to store and slow down the heat flow, the peak indoor temperature will occur at the same time of the day as the peak outdoor temperature.”
Using bricks inside a home can also help moderate internal temperatures even more. For example, a brick feature wall between rooms. This approach can work better internally instead of installing concrete slabs for thermal mass, as slabs are often covered by carpet, timber flooring or furniture, which hinders the movement of heat and its thermal function.
To enhance the effectiveness of internal brickwork, it needs to be located where it has access to a ventilation path in summer to allow for night flushing. “Evening ventilation will allow the mass to store ‘coolth’ so that it will then be able to absorb heat from the rooms all day, moderating summer temperatures,” Inglis says.
All bricks can be a good source of thermal mass, the heavier and denser a brick the more mass it has. “The width of the thermal mass is specific to the climate zone. In areas that have a large diurnal temperature range more thermal mass should be used,” Inglis says.
Externally, rendering a brick wall adds some thickness and will add to the density of thermal mass, albeit only slightly. However, finishing a wall with plasterboard will insulate the mass and prevent it from being able to absorb heat.
“In considering whether or not to use a render, one should also take into account the ongoing maintenance costs associated with render finishes, which require re-painting approximately every seven to 15 years,” says Eddy Maini, marketing manager at Boral Bricks & Roofing. “A face brick finish, on the other hand, is virtually maintenance free and provides a constant solar absorptance, which is not the case with different paint colours. Solar absorptance is the ability of a material to absorb heat energy from sunlight and it changes markedly with different materials and colours.”
All clay bricks have thermal mass, but solid bricks such as pressed bricks have increased density and will therefore have an increased heat capacity. One of the most effective way to use bricks as thermal mass is to use insulated double brick construction for the perimeter walls of a house, with single skin brick construction for internal walls.
There are few restrictions on using bricks in homes, but Mainis says external double brick walls need to incorporate insulation in the cavity to achieve the best performance. He says internal brick walls can also be plastered. “Indeed this is by far the most popular form of house construction in WA.”
To help designers better understand the role of thermal mass in homes, Think Brick recently launched www.designingforclimate.com.au, which simplifies the role of thermal mass in solar passive design and gives tips and advice specific to 27 climatic zones in Australia.
