Basic definition
Thermal mass is a heat storage material, such as water, concrete, masonry, or earthen construction, used in solar thermal energy systems also known as passive solar heating systems. With passive solar heating, the house itself acts as the solar collector and the storage facility. The essential elements of a passive solar system are:
- excellent insulation,
- solar collection (with south-facing windows), and
- thermal storage facilities (thermal mass).
 Thermal mass and insulation
 Types of passive systems
There are three categories that passive systems can fall into:
 Direct gain
One of which is a direct gain system. In this system, large south-facing windows are used to admit the sunlight. and thermal mass is placed in the house to absorb the solar radiation. Another system is indirect-gain.
 Indirect gain
Finally, the third category of these passive solar heating systems is an attached greenhouse on the south side of the house. The greenhouse acts as an expanded thermal storage wall. It will share some of its heat with the adjoining house.
 Thermal mass and R-value
In very limited and specific situations, uncommon during the heating season, thermal mass can marginally increase the apparent R-value of a building assembly such as a wall. Generally speaking thermal mass and R-value are distinct thermodynamic properties and should not be equated. Thermal performance problems apparently seen in some earthship designs may have occurred because of thermal mass being erroneously equated to R-value.
According to the Kansas State University Extension Service the R-value of soil is about 1 per foot.