What is heat capacity?
Heat capacity is the amount of heat needed to raise the temperature of an object or building element by one kelvin (equal to one degree Celsius). The unit is joules per kelvin (J/K). The higher the heat capacity, the more heat an element can absorb and store, and the more slowly it warms up and cools down.
Specific heat
For materials, the specific heat capacity c is given in J/(kg·K): the heat needed to warm 1 kg by one degree. The heat capacity of an element is then C = mass × specific heat capacity.
| Material | Specific heat (J/kg·K) |
|---|---|
| Water | approx. 4,190 |
| Timber | approx. 1,600 |
| Concrete, brick, sand-lime brick | approx. 840 to 1,000 |
| Steel | approx. 450 to 500 |
Timber has a high specific heat per kilogram, but concrete and masonry are far heavier per cubic metre. So a concrete floor stores much more heat per m³ than a timber wall.
In buildings
- Summer comfort: heavy elements (thermal mass) dampen temperature peaks.
- Night ventilation: works best with thermal mass that is cooled at night and absorbs heat by day.
- Underfloor heating: a heavy screed responds slowly but gives even warmth.
- Lightweight construction: in timber frame, mass can be added with gypsum fibre boards, clay plaster or a heavier screed.
Note
Heat capacity is not the same as insulation. A thick concrete wall stores lots of heat but insulates poorly. An energy-efficient home needs both: good insulation and enough thermal mass on the inside.
Related terms
- Accumulation in construction
- Thermal insulation
- Thermal radiation
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