Reading: Sustainable Energy Information Fact Sheet (Thermal Mass) by Sustainable Energy Authority Victoria

Thermal mass is a term used to describe the ability of building materials to store heat (thermal storage capacity).

Building materials that are heavyweight store a lot of heat so are said to have high thermal mass.

By absorbing heat from the atmosphere the internal air temperature is lowered during the day, with the result that comfort is improved without the need for supplementary cooling.

During the night, the heat is slowly released to passing cool breezes (natural ventilation), or extracted by exhaust fans, or is released back into the room itself.

The temperature variation in the brick house is much smaller and temperatures are almost always within the comfort zone.

In general, the greater the daily temperature range, the more thermal mass required.

The benefit of thermal mass are considerably reduced if the external envelope is not insulated. For this reason brick veneer walls offer little thermal mass benefit, as the brick is on the outside of the insulated cavity.

Concrete Slab on Ground

A concrete floor slab directly on the ground will take advantage of the huge thermal mass of the earth benefit.

Inside North Facing Rooms

Using thermal mass in north facing rooms should be a priority, particularly on those walls which receive direct winter sun.

Thermal Mass Locations

Floor: concrete slab or timber floor

External walls: double brick/masonry or brick veneer/weatherboard cladding

Internal walls: masonry or stud walls.

Of all the material choices, the wall selection accounts for about 60 -70% of th thermal mass of the building, with the floor area accounting for about 30 – 40%.

Generally, the more thermal mass the better. A double brick or masonry home on a concrete slab offers the highest comfort benefits and energy savings.

Floor Coverings, Colour & Texture

Carpets

Carpets laid over concrete slab floors tend to insulate the thermal mass of the slab from incoming heat. This delays its entry but also slows down its release. The net result is a temperature rise fo 1 -2 degrees Celsius, which is good in winter, but not so good in summer.

Hard Floor Finishes (great for hotter area’s)

A ceramic tiled finish on a concrete slab floor increases the thermal mass of the floor and the ability to store heat. This can improve cooling in summer (providing the windows are shaded) and works best for rooms with good north solar access.

Colours

Thermal mass that is coloured black absorbs more heat than white coloured material. Its effect on room temperature could be a 2 – 3 Degree’s Celsius year-round temperature rise..

Textures

Shiny or glossy surfaces absorb less heat than dull surfaces.

An exposed brick wall absorbs more heat than a smooth plastered wall.

Light coloured reflective surfaces maximise both daylight and artificial light, whereas dark surfaces absorb light.

Special Construction Types

Mud Brick

Mud Brick and Rammed Earth Homes generally have thick walls (approximately 300mm) and high thermal mass.

When outside temperatures fluctuate above and below comfort temperatures, the high thermal mass of mud bricks considerably reduces heat transfer, performing particularly well in summer.

There will be some heat loss in winter, but if this a problem you can install external insulation which loses the aesthetics of materials.

If uninsulated mud brick or rammed earth is used, limit winter heat losses by keep non – north windows as small as possible.

All north windows should be double glazed and the area of north glazing should be at least 25 – 30% of the floor area, to help heat the building in winter.

Ensure that windows are shaded in summer.

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