Thermal mass

From Wikipedia:


 * Thermal mass  is a concept in building design that describes how the mass of the building provides "inertia" against temperature fluctuations, sometimes known as the   thermal flywheel effect. [1]   For example, when outside temperatures are fluctuating throughout the day, a large thermal mass within the insulated portion of a house can serve to "flatten out" the daily temperature fluctuations, since the thermal mass will absorb thermal energy when the surroundings are higher in temperature than the mass, and give thermal energy back when the surroundings are cooler, without reaching   thermal equilibrium. This is distinct from a material's   insulative  value, which reduces a building's   thermal conductivity, allowing it to be heated or cooled relatively separate from the outside, or even just retain the occupants' thermal energy longer.

Thermal mass and earthships
To achieve a stable year-round temparature, earthships are built with a large thermal mass by making the exterior and some interior walls out of material with a high mass, such as dirt-pounded tires. The floor of the earthship is also directly on the earth's surface, which adds to the thermal mass of the building. In some models the roof is also layered with dirt (e.g. the Simple Survival).

The effect of this design is the thermal mass radiates heat on cool days and conversely absorb heat on hot days, directly into each room.

Contrast to conventional buildings
The relatively large thermal mass of an earthship is a major (if not the major) difference between earthships and conventional buildings. While modern buildings are often insulated, they lack substantial thermal mass, and hence regardless of how good a conventional building's insulation is, they nonetheless need additional fuel (e.g. gas heating) to maintain a stable temperature.

Relationship of insulation to thermal mass
To reduce the influence of outside temperatures on the thermal mass of an earthship, the outer layer of an earthship is insulated. This may include standard building insulation on the roof, outer walls (the insulation is placed on the outside of the outer thermal mass walls) and sometimes under the floor. The greenshouse of an earthship acts as the insulating barrier on the equator-facing side of the earthship while allowing the interior walls and floor (i.e. the majority of the thermal mass of an earthship) to be heated by the winter sun.

The combination of a large thermal mass and insulation of an earthship allows for temperatures variations from summer to winter to be as little as 1 degree with no additional heating/cooling.