Thermal mass is a concept in permaculture that makes use of materials that are capable of storing heat. The material is charged with warmth from the sun, a fire, or some other source, and then slowly releases that warmth over a period of time.
In engineering and physics, this concept is also referred to as heat capacity or the thermal flywheel effect. All materials have some capacity for heat retention. The denser the material, the greater its thermal mass. Very dense materials like rocks, concrete, bricks, and soil have high thermal mass. This makes them harder to heat, but it also makes them stay hot longer.
Thermal mass can be used with any source of heat, and can be used to warm any structure from a home to a greenhouse to a chicken coop.
The most common application for thermal mass involves solar heat. Heat from the sun can be stored in water, in earth, in sand, rocks, and number of other materials. By exposing those materials to the sun's energy during daylight hours, a charge is stored. That charge is then released into the air over the course of the night when ambient temperatures drop.
The Greenhouse of the Future demonstrates an excellent combination of passive solar energy used in conjunction with thermal mass to create a greenhouse that can grow food all year long in far northern climates with no fossil fuels.
Another common use involves rocket mass heaters. In this design, an intense fire is used to charge a mass of cob or stones. The thermal mass is typically shaped into a flat surface for sitting or sleeping, and the seat or bed stays warm for a day or more after the fire is extinguished.
Permaculturists have come up with countless applications for thermal mass. Some creative designs use the sun to warm copper piping on a rooftop. Water is heated within the piping and then circulated into the structure that is to be warmed during the night.
Some designs are as simple as leaving jugs of water in the sun to warm all day and then release that warmth at night. This is a popular tactic in greenhouses and chicken coops.
Jerome Osentowski of the Central Rocky Mountain Permaculture Institute created an innovative design where air is used as a thermal mass. Warm air from the top of a greenhouse is circulated and stored underground during the daytime. Then that warm air is circulated back out to warm the greenhouse at night. Using this design, his greenhouse is able to grow tropical plants all winter at elevation in the Rocky Mountains. He details the design at length in his book The Forest Garden Greenhouse.
To learn more about thermal mass in greenhouses, check out the Greenhouse of the Future video.
If you want to learn more about thermal mass as it's used in rocket mass heaters, watch Paul Wheaton's videos DIY Rocket Mass Heaters or read the book The Rocket Mass Heater Builder's Guide by Ernie and Erica Wisner.