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).

Good thermal mass is an important part of passive solar and energy efficient building design, and contributes to an energy efficient building.

Thermal mass and insulation[edit | edit source]

Where thermal mass is used in walls, e.g. stone, strawbale or earth construction, these thick walls also provide significant insulation. See insulating walls.

Types of passive systems[edit | edit source]

There are three categories that passive systems can fall into:

Direct gain[edit | edit source]

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[edit | edit source]

Indirect gain collects and stores the solar energy in one part of the building and uses natural heat transfer to distribute the heat throughout the building, such as the Trombe wall.

Greenhouse[edit | edit source]

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[edit | edit source]

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.

See also[edit | edit source]

  • Earth sheltering

External links[edit | edit source]

FA info icon.svg Angle down icon.svg Page data
Part of PH261
Keywords passive solar, thermal insulation, energy efficiency, construction, passive solar, energy efficient building
SDG SDG07 Affordable and clean energy
Authors Suzann.pomraning, KVDP, Chris Watkins
License CC-BY-SA-3.0
Organizations Clarion University
Language English (en)
Related 0 subpages, 32 pages link here
Impact 716 page views
Created October 10, 2007 by Suzann.pomraning
Modified May 28, 2024 by Kathy Nativi
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