Wall Thermal Mass Calculator

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), meaning the energy needed to raise 1 kg by 1 Kelvin
• Volume = Wall Area × Wall Thickness

The result is in Joules per Kelvin (J/K).
That is the total energy required to raise the entire wall mass by one degree.

Typical values for common wall materials:

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, extra material stops helping. Heat never penetrates to the inner layers within a 24-hour cycle, so those layers sit at a steady temperature and take no part in the daily swing. The effective depth is roughly:

• Concrete: up to ~150-200 mm (6-8 inches)
• Brick: up to ~100-150 mm (4-6 inches)

The calculator does not cap you at those depths, because the full figure is the right answer to a different question. A 400 mm rammed-earth wall really does hold that much heat, which matters for a seasonal or multi-day swing. It just will not all be available to you between one afternoon and the following morning, so read anything past about 200 mm as storage you are paying for and not using on a daily cycle.

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.