The Principle of Thermal Mass

sustain-3-passive-110405165024-phpapp02.pptThermal mass is a vital and complimentary component of passive solar design.
A material that has thermal mass is one that has the capacity to absorb, store and release the sun’s heat energy. Its density and levels of conductivity help to keep the internal temperature of a building stable. Objects that have thermal mass have inherent qualities for both heating and cooling.

Basic Passive Solar Design
Materials with thermal mass are typically used in the floor or inside walls of a passive solar structure and located near the solar glazing (southern facing windows) to allow the sun’s energy to shine to directly on them.  In this manner, they can store and release the sun’s heat energy
They are generally dense materials, such as concrete, stone, brick or ceramic tile. In the diagram to the right, the thermal mass absorbs and distributes heat energy.
Common building materials with thermal mass include stone, tiles, and concrete.
Water also is very dense and has a high capacity to store and release heat. It, however, is more difficult to build into the design of houses.

Chart of some thermally massive materials

Chart of some common thermally massive materials.

Thermal Mass and Heating
When thermal mass is used for its heating capacity, when exposed to sunlight, the thermal mass absorbs the sun’s heat energy.
Because objects with thermal mass are typically dense, they have the ability to conduct heat. Because of this, all of the thermal mass does not need to be exposed to the direct sunlight, but a large portion of it should be. The heat energy that is absorbed will slowly spread (conductivity) through the mass and then radiate or release the heat energy throughout the evening and night.

Thermal Mass and Cooling
Objects with Thermal Mass will also keep a building cool. Thermal mass can be used for its capacity keep a structure cool during the hotter summer months.
Have you ever felt the coolness of marble on a hot summer day?  If dense objects with thermal mass are kept out of the sun, they will tend to stay cool and thus, help to keep the building cool.  I was in a hot, humid city when I went into a building and leaned up against the cool marble.  While it was hot and muggy outside, because the marble was in the shade, it was cool.
The summer sun’s heat energy can and should be blocked from entering the building and hitting the thermal mass, by an overhang or other type of device or entry way.
In the evening, when it is cooler outside, a passive solar building can be opened up to absorb the cooler evening and night temperatures within its mass. The dense material can cool and will absorb heat the following day.

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