How to calculate the thermal mass needed for my space?

Estimating Space Volume

To start, calculate the volume of the space you’re working with. This can be done by multiplying the length, width, and height of the space. For example, a room with dimensions 10 feet by 12 feet by 8 feet would have a volume of 960 cubic feet. Next, multiply this volume by a density factor, typically between 3 and 6 pounds per cubic foot of air space. A common rule of thumb is to use 4-4.5 pounds per cubic foot for a moderate climate.

Choosing Thermal Mass Materials

Thermal mass materials can include materials like concrete, brick, or even metal. When choosing a thermal mass material, consider factors like durability, cost, and ease of installation. For a rocket mass heater, a common material choice is refractory concrete, which can be made with a mixture of Portland cement, silica sand, and vermiculite. A typical recipe for refractory concrete includes 1 part cement to 2 parts sand to 1 part vermiculite.

Calculating Thermal Mass Needs

To calculate the total thermal mass needed, multiply the volume of the space by the density factor, and then divide by the desired thermal mass density of the material. For example, if you have a 960 cubic foot space with a desired thermal mass density of 4 pounds per cubic foot, and you’re using a refractory concrete with a density of 80 pounds per cubic foot, you would need 48 cubic feet of refractory concrete.