Thermal mass refers to the ability of large quantities of certain
dense materials to readily absorb the heat energy transferred to them.
Materials with notably high thermal mass include water, earth, and cement,
to name just a few. These materials store heat energy much like a battery.
In the case of an earthship, the sun's energy is absorbed by the densely-
packed earth-rammed walls throughout the day, and is then in turn slowly
released back into the dwelling at night as the temperature outside of
the walls falls below equilibrium. This action has a stabilizing effect
upon room temperatures, keeping them somewhere in the habitable range of
60-70 degrees under the most optimal earthship designs.
Thermal mass is NOT to be confused with insulation.
As an insulator,
earth, water, and cement are not very effective. Whereas thermal mass
readily absorbs and releases heat energy, insulation prevents or greatly
retards the transfer of heat energy from one place to another. As such,
insulation is virtually the opposite of thermal mass. It is the combined
use of thermal mass and insulation however that make contemporary earthship
designs so effective.