J/kgKSpecific heat capacity, c
What it is. The energy required by one kilogram of material to raise its temperature by 1 °C.
How to read it. The higher it is, the more heat the material stores per kilogram. It says nothing, however, about how well it insulates: that is indicated by λ and R.
kJ/m²KHeat capacity of the layer
What it is. How much heat one m² of the layer stores when its temperature rises by 1 °C: density × c × thickness.
How to read it. Heavy, thick materials store a great deal; lightweight materials store little. Calculation examples using the values in the table:
- Concrete, 15 cm (2 400 kg/m³): 2 400 × 1 000 × 0.15 = 360 kJ/m²K.
- Plasterboard, 12.5 mm (900 kg/m³): 900 × 1 000 × 0.0125 ≈ 11 kJ/m²K.
hoursTime lag
What it is. The delay between the warmest moment outside and the warmest moment on the inner face of the wall or roof.
How to read it. A longer time lag shifts the heat peak towards the evening or night, when it can be removed by ventilation. It depends on the mass, the specific heat capacity and the λ of all layers, not of a single material. It is calculated according to SR EN ISO 13786.
Advantages and disadvantages
Advantages
- In summer, thermal mass dampens and delays heat peaks.
- The room temperature varies less over the course of the day.
- Heat from the sun or from the heating system is stored and released gradually.
Disadvantages
- The room is slow to heat up, particularly when it is used only occasionally, for example a holiday home.
- After several days of heatwave, the charged mass releases heat towards the interior during the night as well, if it cannot be ventilated.
- It does not replace insulation: in winter, heat losses are reduced by the thermal resistance R, not by mass.
- Damp material stores more heat, but also conducts it more readily; SR EN ISO 10456 corrects λ according to moisture content.
Typical values of specific heat capacity
| Material | Density, kg/m³ | c, J/kgK |
|---|
| Water, at 10 °C | 1 000 | 4 190 |
| OSB | 650 | 1 700 |
| Structural timber | 500 | 1 600 |
| Expanded polystyrene | 10–50 | 1 450 |
| Mineral wool | 10–200 | 1 030 |
| Air | 1,23 | 1 008 |
| Concrete | 2 400 | 1 000 |
| Plasterboard | 900 | 1 000 |
| Aluminium, alloys | 2 800 | 880 |
| Steel | 7 800 | 450 |
Source: tabulated design values, EN 12524:2000, Tables 1 and 2, adopted in SR EN ISO 10456.
Reflective membranes and Wolle are lightweight materials: they have little mass per m² and therefore store little heat. Their role in the build-up is to stop the passage of heat, through R and through emissivity, not to store it.
