Wall Thermal Mass Calculator
Calculate the thermal mass of a wall assembly to understand heat storage capacity.
Useful for passive solar and energy-efficient building design.
Thermal mass is the ability of a material to absorb, store, and slowly release heat. High thermal mass materials (like concrete, brick, and rammed earth) heat up slowly during the day and release heat slowly at night, buffering temperature swings. This is the basis of passive solar building design.
The formula for thermal mass: Thermal Mass = Density × Specific Heat Capacity × Volume
Where:
- Density is in kg/m³ (or lb/ft³)
- Specific Heat Capacity is in J/(kg·K) — how much energy is needed to raise 1 kg by 1 Kelvin
- Volume = Wall Area × Wall Thickness
The result is in Joules per Kelvin (J/K) — the total energy required to raise the entire wall mass by 1 degree.
Typical values for common wall materials:
| Material | Density (kg/m³) | Specific Heat (J/kg·K) |
|---|---|---|
| Concrete (standard) | 2,300 | 880 |
| Brick (clay) | 1,800 | 840 |
| Rammed earth | 2,000 | 840 |
| Adobe | 1,600 | 920 |
| Timber / wood | 600 | 1,700 |
| Lightweight concrete | 1,400 | 840 |
| Stone (granite) | 2,700 | 790 |
Why thermal mass matters: In passive solar buildings, thick thermal mass walls on the south side (northern hemisphere) or north side (southern hemisphere) absorb solar radiation during daylight hours and radiate that heat back into the living space at night. This can significantly reduce heating and cooling costs.
Effective thermal mass thickness: Beyond a certain thickness, additional material does not help — heat simply never penetrates to the inner layers in a 24-hour cycle. The effective depth of thermal mass is approximately:
- Concrete: up to ~150–200 mm (6–8 inches)
- Brick: up to ~100–150 mm (4–6 inches)
Thermal mass vs. insulation: These serve different functions. Insulation (R-value) slows the rate of heat transfer. Thermal mass stores heat. In moderate climates with large day-night temperature swings, high thermal mass + moderate insulation is often more effective than high insulation alone.
Units note: Metric (SI) units are standard for thermal calculations. Imperial volumes in ft³ × 0.0283168 = m³.