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October 16, 2016
Thermal mass is the ability of a material to absorb and store 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 have low thermal mass. Appropriate use of thermal mass throughout your home can make a big difference to comfort and heating and cooling bills.
Thermal mass can store solar energy during the day and re-radiate it at night.
Thermal mass, correctly used, moderates internal temperatures by averaging out diurnal (day−night) extremes. This increases comfort and reduces 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 to you all night as you attempt to sleep during a summer heatwave or absorb all the heat you produce 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:
Thermal mass acts as a thermal battery. During summer it 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; insulation stops heat flowing into or out of the building. A high thermal mass material is not generally a good thermal insulator (see Rammed earth).
Thermal mass is particularly beneficial where there is a big difference between day and night outdoor temperatures.
Daily temperature fluctuations for different construction methods.
Correct use of thermal mass can delay heat flow through the building envelope by as much as 10−12 hours, producing a warmer house at night in winter and a cooler house during the day in summer (Wilson 1998).
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.
Allow thermal mass to absorb heat during the day from direct sunlight or from radiant heaters. It re-radiates this warmth back into the home throughout the night.
Allow cool night breezes and/or convection currents to pass over the thermal mass, drawing out all the stored energy. During the day protect the thermal mass from excess summer sun with shading and insulation if required.