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Thermal mass
The term “thermal mass” refers to a material’s ability to absorb and store heat. Dense materials such as concrete take a long time (and a lot of energy) to heat up and cool down. By contrast, lightweight, low-density materials that contain a lot of air, such as wood, absorb and lose heat much more quickly.
In fact, it can take up to 12 hours for something like concrete to release all of its stored heat. And that means a concrete floor that warms up during the day will still be releasing heat into your home long into the night.
Correct use of thermal mass can make your home more comfortable – so it is warm or cool when you want with less need for heating or air-conditioning. That will help reduce your energy use and bills. On the other hand, using building materials with a lot of thermal mass in a hot tropical climate could stop your home cooling down at night.
Thermal mass of different building materials
Concrete, sandstone, compressed earth blocks and rammed earth are all dense materials with a high thermal mass. Low-density materials such as AAC blocks and timber have lower thermal mass. They heat up and cool down more quickly. Brick is somewhere in the middle.
Thermal mass and your climate
Thermal mass will be most helpful in climates with sunny days and cooler nights in winter. On the other hand, in tropical regions with hot sticky nights you should use material with low thermal mass. This will let the heat escape more quickly so your home will be cooler during the night.
As a rule, if you need energy mainly to heat your home in winter, you’ll benefit from high thermal mass. But if you use more energy for air-conditioning to cool your home in summer, you want to avoid thermal mass.
Another measure is called diurnal range – the variation between day and night temperatures. The greater the temperature variation, the more your home will benefit from thermal mass to absorb heat in the day and release it at night.
• Temperate: (cooler winter nights): high thermal mass materials release heat slowly to provide warmth on winter nights. Good passive solar design is needed to allow heat to hit your floor in winter but not in summer.
• Tropical: (warm nights all year): low thermal mass materials release heat quickly in the day, to give you cooler nights. Good air circulation is needed to blow the warm air out of the house quickly.
Locating thermal mass
Thermal mass must be inside your home’s “thermal envelope” – that is, inside the building’s insulation. Roof tiles, or the external brick wall of a brick veneer house, are both on the outside of any insulation so the heat escapes into the outside air, not your home. They provide no thermal mass for your home. If you want walls to provide thermal mass, you need reverse brick veneer (with the bricks on the inside, within the insulation) or double brick. The most obvious area for thermal mass is your floor. Heat rises, so heat escaping from the floor will rise directly into your home.
The most common flooring to provide thermal mass is a concrete slab. But note that in cool climates you should insulate the edges of the slab to prevent it losing heat to the surrounding earth.
You can add extra thermal mass with internal walls made from concrete or brick. You can also use water, which has high thermal mass, with features such as an indoor pool, or even large pots full of water, positioned in a sunny spot.
Other factors
The amount of heat a floor can store depends on the surface area of floor exposed to sunlight, and its thickness. The more surface area is exposed to sunlight, and the thicker the floor, the more heat it will store. The other factor is floor covering. Carpets, by insulating the floor, reduces the amount of heat it absorbs.
Thermal mass and passive solar design
Thermal mass should be used in conjunction with passive solar design and shading. This will ensure sunshine hits your home’s floor or internal walls in summer but is screened out by eaves, verandas, awnings and other shade/sail devices during summer.
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Building
Heating Systems
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