How to use thermal mass

G Magazine

Artificial heating and cooling have a significant eco-footprint, but what if there was a way to regulate temperature naturally? Find out how thermal mass works.

Thermal mass

In summer, thermal mass is the passage of heat through the walls of a home. This 'thermal lag' delays outdoor heat reaching the inside of a home as it remains trapped in the building structure, therefore postponing or even eliminating the need for air conditioning. Rough, dark walls and floors will absorb more heat than their smooth, light counterparts, so consider creating a feature wall next time you renovate.

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Consistency equals comfort. In winter we heat our homes and in summer we cool them, in each case to achieve a comfortable living temperature. In environmental terms, this isn't such a happy medium, with artificial heating and cooling generating large amounts of greenhouse gas emissions.

The Department of Climate Change and Energy Efficiency estimates that heating and cooling comprises up to 20 per cent of total household energy use, while Sustainability Victoria reports that for Victorian households, 50 per cent of household energy use is spent on heating alone.

But what if there was a way to maintain a comfortable room temperature inside our homes all year round while reducing artificial climate control? The answer lies in a building technique that proponents say can reduce total household heating and cooling energy requirements by up to 25 per cent.

Introducing thermal mass

A buzz term circulating around the renovation industry is thermal mass. In essence, it refers to the ability of a material to store and release heat. In much the same way as a sponge works, materials with high thermal mass absorb, store and slowly re-radiate heat back into a room, maintaining a comfortable temperature. This, consequently, levels day and night extremes and limits the need for heating and air-conditioning.

Density is the key. Bricks, concrete and tiles have high thermal mass as their dense, heavy structure allows for substantial heat retention. On the other hand, lightweight materials like timber have low thermal mass, as they require less energy to heat up, and lose energy more quickly.

Summer sun

In summer, thermal mass limits the passage of heat though the walls of a home. This 'thermal lag' (the
time taken for heat to reach the inside a home) delays outdoor heat reaching the inside of a home as it remains trapped in the building structure, therefore postponing or even eliminating the need for air-conditioning.

A 2006 study by Austral Bricks found that cavity (double) brick homes with very high thermal mass had a thermal lag of 7 to 8 hours. Brick veneer (single brick) dwellings of high thermal mass reported a lag of 5 to 6 hours, and houses made of lightweight materials with low thermal mass reported virtually no lag.

Combining materials of high thermal mass with good ventilation is crucial in summer. Technical advisor for industry group Think Brick Australia, Cathy Inglis, explains that once the thermal lag time has passed, the heat stored in the thermal mass is re-radiated back into the house.

"That's why you need a combination of ventilation and thermal mass because you need to open windows and doors to let the heat back out so the thermal mass cools down and is ready to absorb the next day's heat."

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