Thermal mass

Thermal mass describes the ability of a material to absorb and store heat. Used correctly, it moderates internal temperatures by averaging out day-night extremes, making your house more comfortable and reducing your energy costs.

How thermal mass works

Thermal mass works like a thermal battery.

  • In summer - it absorbs heat during the day and releases it by night, keeping your house cooler during the day.
  • In winter - it stores heat from the sun or heaters around your home and radiates it at night, helping your house stay warm.

Be aware that using too much thermal mass may mean your home never gets warm so be careful to get the balance right.

Concrete floor slabs

The simplest and most cost-effective way to use thermal mass is with a concrete floor slab. It should be:

  • exposed to the sun in winter but shaded during summer
  • 100 mm thick and well insulated underneath and around the footings and perimeter
  • uncovered or tiled in areas exposed to the sun (you can polish or paint the concrete, preferably dark)
  • covered or carpeted in rooms that don't collect sun.

If you can’t use a concrete slab, use a suspended concrete floor in rooms with north-facing windows.

Masonry walls

You can also use internal or external masonry walls as thermal mass. To be effective, they must be exposed to sun coming in through north-facing windows.

  • External masonry walls - must be well insulated on the outside, with the mass exposed to the inside of the house (without internal lining, but it can be plastered, painted or wall-papered).
  • Internal masonry walls - don't lose heat to the outside so don't need insulating. They can also conduct heat to the rooms behind the wall that don't get direct sunlight.