Well-placed, high-density materials can keep your home comfortable.
By and large, when house designers mention “thermal mass“, their clients’ minds go blank. The topic seems as impenetrable as a thick bluestone wall.
Sustainable building expert Dr Chris Reardon, of Suntech Design, says it’s a crucial concept for people to grasp – and it’s actually not so complex. “Thermal mass is the battery of passive heating and cooling,” he explains. “It’s where you store free heat from the sun, or free cooling from breezes or radiation to clear night skies.”
Dense building materials, such as concrete, brick, earth or tiles, have high thermal mass – they need a lot of heat energy to warm them up and they retain the heat for a long time. “The main value of thermal mass is to even out day/night temperature ranges. In winter, it stores daytime solar heat and augments night temperatures. In summer, stored night ‘coolth’ can absorb daytime heat gains,” Dr Reardon says. In contrast, lightweight materials heat and cool rapidly.
High-density materials are helpful in climates where there’s a gap of more than six degrees between daytime maximums and night-time minimums. In Melbourne, the temperature often varies by 10 degrees or more. “The more extreme the climate, the more thermal mass you need,” he says.
But mass is just one element of good solar passive design. If not co-ordinated with the orientation, windows, shading and insulation, the heavy materials can accentuate the worst aspects of the climate – the home could absorb extra warmth in summer and stay colder in winter.
“Thermal mass must be exposed on the inside of the house and insulated from the outside,” Dr Reardon explains. For example, the timber and plasterboard internal wall in a brick veneer home insulates the mass in the bricks, meaning they don’t help to curb temperature extremes. The mass is better put to use in reverse brick veneer construction, or by designing internal feature walls of exposed brick, cement-rendered brick or earth.
According to Riccardo Zen, from Zen Architects in North Fitzroy, carefully placed high-density materials are essential to cut energy needs for homes in Victoria. “Our basic goal is to eliminate heating and cooling in our buildings,” he says. “It’s very hard to do that unless you have some form of mass.”
A classic example of thermal mass is an exposed concrete slab floor, positioned in front of windows in a north-facing living room. In the winter, the sun can shine directly on the slab, which absorbs the radiation and warms the house into the night. With appropriate shading, sunlight won’t hit the concrete in summer and the chill of the slab will help the home stay cool.
You don’t have to start from scratch to improve the thermal mass in your home. Mr Zen says building designers are learning nifty ways to add high-density materials to existing houses, including suspended slabs, aerated concrete blocks and tiles. “We did a project where we brought concrete pavers onto a timber floor using the existing joists,” he says.
“People are also experimenting with using water tanks. Water is one of the best forms of thermal mass – you can also shift it and put it back seasonally when it’s required,” Mr Zen says. “We’re becoming more sophisticated in the way we use mass. There are some hybrid systems evolving to tackle difficult sites.”