Thermal mass is described as the ability of a material to absorb and store heat energy. High density materials such as concrete, bricks and tiles require a lot of heat energy to change their temperature, and are therefore said to have high thermal mass. On the other hand, lightweight materials such as timber have low thermal mass.

When appropriately used, thermal mass can make a big difference to the comfort factor throughout your home as well as your heating and cooling bills.

Thermal mass can store solar energy during the day and re-radiate it at night. When correctly used, thermal mass can moderate internal temperatures by averaging out diurnal extremes, thereby increasing comfort and reducing energy costs. However, poor use of thermal mass can make matters worse and become a huge liability for both energy consumption and comfort. It can radiate heat all night during a summer heatwave or absorb all the heat produced on a winter night.

To be effective, thermal mass must be integrated with sound passive design techniques. This means having appropriate areas of glazing facing appropriate directions with appropriate levels of shading, ventilation, insulation and thermal mass.

How thermal mass works:

During summer thermal mass absorbs heat during the day and releases it by night to cooling breezes or clear night skies, keeping the house comfortable. In winter the same thermal mass can store the heat from the sun or heaters to release it at night, helping the home stay warm.

Thermal mass is not a substitute for insulation: Thermal mass stores and re-releases heat while insulation stops heat flowing into or out of the building. A high thermal mass material is not generally a good thermal insulator; thermal mass is particularly beneficial where there is a big difference between day and night temperatures in the outdoor environment.

A high mass building needs to gain or lose a large amount of energy to change its internal temperature, whereas a lightweight building requires only a small energy gain or loss to change the air temperature. This is an important factor to consider when choosing construction systems and assessing climate change adaptation.

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