Thermal mass is any material that stores heat by getting warmer, and the heat it holds is its mass times its specific heat times its temperature rise. A cubic metre of water warmed by 10 K holds 41.8 MJ (11.6 kWh) and a cubic metre of solid stone holds about 21 MJ, so water stores about twice as much per volume. The mass only helps if it sits inside the insulated part of the building, where its stored heat is released back into the living space.
Part of Heat Storage Systems
Thermal mass is the heat-holding capacity of a material, which smooths out the gap between when heat is available and when it is needed.
Heat Stored Is Mass × Specific Heat × Rise
Specific heat is the energy needed to warm 1 kg of a material by 1 K. The heat stored is Q = m × c × ΔT. The parent page gives specific heats of 4.18 kJ/kg·K for water and 0.84 kJ/kg·K for stone, and densities of 1,000 and 2,500 kg/m³. Per cubic metre, that is 4.18 MJ per kelvin for water (4.18 kJ/kg·K × 1,000 kg/m³) and 2.10 MJ per kelvin for stone (0.84 × 2,500).
Worked for 1 m³ warmed by 10 K:
- Water: 4.18 MJ/m³·K × 10 K = 41.8 MJ = 41.8 ÷ 3.6 = 11.6 kWh.
- Stone: 2.10 × 10 = 21.0 MJ, about 21 MJ, = 5.8 kWh.
Per kilogram, water holds 4.18 ÷ 0.84 = 5.0 times as much as stone; per cubic metre, about twice, because stone is 2.5 times as dense (Heat Capacity of Common Materials compares the two bases carefully).
Three Ideas Make It Useful
- Charge and discharge. Heat goes in when you have it (a fire, the sun) and comes out when you need it (Sensible vs Latent Heat).
- Insulation and mass work together. Mass inside the insulated envelope gives its heat to the room; mass outside gives it to the weather (Insulation vs Mass Trade-offs).
- Time lag. A thick mass releases heat slowly, which is why a masonry heater warms a room for half a day after the fire is out (Retained-Heat Structures).
The wiki’s house-design pages apply the same idea to a passive-solar room (Thermal Mass Principles).
What You Need
- A mass: water in tanks, rock, brick, cob or concrete
- Insulation on the outside of the mass
- A way to charge it: a fire, hot gas, a solar collector
- A way to read it: a thermometer, or the hand on a surface (carefully)
Common Mistakes
- Adding mass without insulation. The heat leaks out as fast as it goes in.
- Counting heat that can’t be used. Only the part of the mass above room temperature gives heat to the room.
- Mixing up kilograms and cubic metres. The comparison changes by a factor of 2.5 depending on the basis.
Safety
Stored heat burns long after the fire is out
A hot tank, a heated stone bed or a masonry heater stays hot for hours, and hot water scalds in seconds at 60°C or above. Surfaces above about 60°C burn with brief contact. Guard children and the elderly, who are injured at lower temperatures and shorter exposures than adults, and treat burns with cool running water (Thermal Injury Care).
What’s Next
- Learn next: Heat Capacity of Common Materials — the numbers for each material
- Combine with: Where to Insulate vs Where to Mass — placing mass in a house