Design Considerations / Comfort & Energy / Construction & materials
Thermal mass
Heavy materials like concrete, brick, stone and earth soak up heat and give it back slowly. Used well, they store the winter sun for the evening and the summer night's cool for the next day. Used badly, they just take longer to heat.
Design ConsiderationsComfort & EnergyAt a glance
Heavy materials store heat and release it slowly. In Canberra, with its big gap between day and night temperatures, a sunlit, uncovered slab or brick wall inside an insulated shell stores free winter heat for the evening and summer night cool for the next day.
- Concrete stores about one and a half times the heat of brick, and nearly four times that of lightweight aerated concrete.
- Only about the first 100 mm takes part in a daily cycle, so exposed area matters more than thickness.
- 20 m² of sunlit slab warming by 3°C stores about 3.4 kWh, roughly an hour and three-quarters of a 2 kW heater.
- Mass pays off where days and nights differ by more than about 10°C. Canberra's swing is 11 to 15°C.
- Put it where the winter sun lands, inside the insulation, and leave it uncovered. Keep it out of shaded rooms.
- More north glass isn't automatically better. Size the glass and mass together.
- In summer, shade the mass and flush it with cool night air.
The ideaA heat battery
Thermal mass is a material's ability to store heat. A heavy floor in winter sun warms slowly through the day, then gives that warmth back to the room after sunset. In summer, the same floor cooled by night air soaks up heat during the day and keeps the room cooler. It acts like a flywheel, evening out the swings between day and night.
Mass is not insulation. Insulation slows heat moving through a wall, and mass stores it. A solid concrete or earth wall with no insulation stores heat well, but it also loses it steadily to the cold outside.
How much heat each material holds, per cubic metre for each degree:
| Material | Heat capacity (kJ/m³·K) |
|---|---|
| Water | 4,186 |
| Concrete | 2,060 |
| Sandstone | 1,800 |
| Rammed earth | 1,673 |
| Brick | 1,360 |
| Mud brick (adobe) | 1,300 |
| Autoclaved aerated concrete (AAC) | 550 |
Concrete holds about one and a half times as much heat as the same volume of brick, and nearly four times as much as lightweight aerated concrete. Water holds twice as much as concrete.
How muchSurface matters more than thickness
In a single day-and-night cycle, heat only gets about 100 mm into concrete or masonry. Mass deeper than that barely takes part. A 100 mm slab and a 300 mm slab behave much the same day to day, but twice the exposed area stores twice the heat.
To give a sense of scale: 20 m² of sunlit slab, with the top 100 mm warming by 3°C during a winter day, stores about 12,000 kJ, or roughly 3.4 kWh. It releases that heat into the room over the evening. That's about what a 2 kW heater delivers in an hour and three-quarters, free, every sunny winter day.
The heat is released slowly. Peak temperatures in a heavy room arrive hours after the sun has gone, which is exactly when a Canberra living room needs them.
ClimateWhen mass helps
The size of the daily temperature swing decides how useful mass is:
| Average gap between day and night temperature | Recommended construction |
|---|---|
| 6°C or less | Lightweight performs best |
| 6 to 10°C | Moderate mass, such as a slab on ground with brick veneer |
| Over 10°C | High mass is desirable |
Canberra's swing is about 15°C in summer and 11°C in winter. That puts it firmly in the band where heavy construction pays off. In cool climates mass helps whatever the swing, provided the house is well insulated and airtight. A heavy house that leaks heat just takes longer to warm up.
PlacementWhere mass works
- Where the winter sun lands. Ground-floor, north-facing living areas are the best place: the slab under the north windows, and the internal walls the sun reaches.
- Inside the insulation. Mass must sit on the warm side of the insulated shell. Reverse brick veneer, internal brick or block walls, and an insulated slab all do this.
- Uncovered. Carpet, cork and timber flooring insulate the slab from the room. Keep sunlit areas as tiles, polished concrete or stone, and put rugs in the shade. Don't line masonry walls with plasterboard on battens, which hides the mass behind an air gap.
- Darker finishes in sunlit areas absorb more of the sun's heat than pale ones.
- Not in shaded rooms. Mass in a south-facing or poorly insulated room just stays cold and takes longer to heat. Upstairs bedrooms generally don't need it.
- Insulated underneath and at the edges. In Canberra, insulate slab edges, and ideally the underside, so the stored heat goes into the room rather than the ground.
More north glass isn't more heat
It's tempting to think that if some north glass is good, more is better. Modelling of a house in a cool climate shows otherwise. As north glass grows, heating needs fall at first, then rise again beyond a fairly modest window area, because the extra glass loses more heat overnight than it gains by day. With very large north windows, the room ran about 3°C hotter on a cool, sunny day. The glass and the mass have to be sized together. The right balance is best found by modelling the actual design.
SummerShade it and cool it at night
Mass works in summer too, but only if it's kept out of the sun and cooled overnight:
- Shade the glass so summer sun doesn't reach the floor. The eave that lets winter sun onto the slab should keep summer sun off it.
- Flush the house at night. Open windows on opposite sides, or high and low, when the evening air cools. Canberra's summer nights are usually cool enough to draw the day's heat out of the slab.
- Close up in the morning and keep the cool in through the day.
Unshaded mass is the worst of both worlds. It soaks up the afternoon sun and radiates it back all night.
AlternativesOther ways to add mass
- Water stores twice as much heat as concrete by volume. Sealed tanks or containers in a sunlit space are an unusual but effective option.
- Phase-change materials absorb heat as they melt, at around 22 to 25°C, and release it as they set. A 13 mm phase-change plasterboard can hold roughly as much heat as 50 mm of concrete, which makes it useful in lightweight houses and renovations.
- Earth construction, such as rammed earth and mud brick, provides high mass with lower processing energy, though cement-stabilised rammed earth adds embodied carbon.
Concrete and brick carry significant embodied emissions. Use mass where it does the most work, in sunlit living areas, rather than everywhere by default. Recycled bricks are a lower-impact choice for internal walls.
Applying it locallyCanberra and New South Wales
Canberra and the Southern Tablelands. This is ideal territory for mass: big daily swings, sunny winters and cool summer nights. Put heavy floors and internal walls in the north living areas, inside a well-insulated shell.
Alpine areas. Mass helps only where it gets direct sun. In rooms without sun, use well-insulated lightweight construction that warms quickly.
The NSW coast. Daily swings are smaller, so moderate mass, such as a slab with lightweight walls, usually suits better than heavy construction.
Western plains. Large swings and hot summers favour high mass with high insulation, shaded by day and flushed at night.