Thermal mass

See Specific heat capacity for discussion of the atomic mechanisms which cause the heat storage abilities of pure sustances to vary.

Thermal mass ($C_{\mathrm {th} }$, also called heat capacity) is the capacity of a body to store heat. It is typically measured in units of J/°C or J/K (which are equivalent). If the body consists of a heterogeneous material with sufficiently known physical properties, the thermal mass (or heat capacity, an extensive property) can be calculated as the product of the mass $m$ of the body (or some other measure of the amount of material, such as number of moles of molecules which are present) and the specific heat capacity $c$, (an intensive property) for the material. Here the specific heat capacity is expressed in terms of a mass or number of moles or some other measure of the amount of material, and must be multiplied by similar units to give the heat capacity of the entire body of material. For bodies made of many materials, the sum of heat capacities for their pure components may be used in the calculation, or in some cases (as for a whole animal, for example) the number may simply be measure for the entire body in question, directly.

Thermal mass as a concept is most frequently applied in the field of building design. In this context, thermal mass provides 'inertia' against temperature fluctuations. For example, for a building, when outside temperatures are fluctuating throughout the day, a large thermal mass within the insulated portion of the house can serve to 'flatten out' the daily temperature fluctuations, since the thermal mass will absorb heat when the surroundings are hotter than the mass, and give heat back when the surroundings are cooler.

Thermal mass may also be used for bodies of water or any other structure or body in engineering or biology.

Thermal mass in buildings

Thermal mass is effective in improving building comfort in any place that experiences these types of daily temperature fluctuations—both in winter as well as in summer. When used well and combined with passive solar design, thermal mass can play an important role in major reductions to energy use in active heating and cooling systems and hence the reduction of greenhouse gas emissions due to fossil fuel burning in power stations.

Properties required for good thermal mass

Ideal materials for thermal mass are those materials that have:

• high specific heat capacity,
• high density

Any solid, liquid, or gas that has mass will have some thermal mass. A common misconception is that only concrete or earth soil has thermal mass; even air has thermal mass (although very little.)

A useful table of volumetric heat capacity for building materials is available from yourhome.gov.au (but note that their definition of thermal mass is slightly different).

Use of thermal mass in different climates

The correct use and application of thermal mass is dependent on the prevailing climate in a district.

Temperate / Cold Temperate climates

Thermal mass is ideally placed within the building and situated where it still can be exposed to winter sunlight (via windows) but insulated from heat loss.

The thermal mass is warmed passively by the sun or additionally by internal heating systems during the day. Heat stored in the mass is then released back into the interior during the night. It is essential that it be used in conjunction with the standard principles of passive solar design.

Any form of thermal mass can be used. A concrete slab foundation either left exposed or covered with conductive materials e.g. tiles; is one easy solution. Another novel method is to place the masonry facade of a timber-framed house on the inside ('reverse-brick veneer'). Thermal mass in this situation is best applied over a large area rather than in large volumes or thicknesses. 7.5-10 cm (3-4") is often adequate.

Since the most important source of heat is from the sun, the ratio of glazing to thermal mass is an important factor to consider. Various formulas have been devised to determine this. As a general rule, additional solar-exposed thermal mass needs to applied in a ratio from 6-8:1 for any area of north facing (Southern Hemisphere)(south facing, Northern Hemisphere) glazing above 7% of the total floor area. e.g. a 200 sqm house with 20sqm of north facing glazing has 10% of glazing by total floor area. 6sqm of that glazing will require additional thermal mass. Therefore, 36-48 sqm of solar-exposed thermal mass is required. The exact requirements vary from climate to climate.

Hot, arid climates (e.g. desert)

This is a classical use of thermal mass. Examples include adobe or rammed earth houses. Its function is highly dependent on marked diurnal temperature variations. The wall predominantly acts to retard heat flow from the exterior to the interior during the day. The high volumetric heat capacity and thickness prevents heat from reaching the inner surface. When temperatures fall at night, the walls re-radiate the heat back into the night sky. In this application it is important for such walls to be massive to prevent the ingress of heat into the interior.

Hot humid climates (e.g. sub-tropical/tropical)

The use of thermal mass is the most challenging in this environment where night temperatures remain elevated. Its use is primarily as a temporary heat sink. However, it needs to be strategically located to prevent overheating. It should be placed in an area that is not directly exposed to solar gain and also allow adequate ventilation at night to carry away stored energy without increasing internal temperatures any further. If to be used at all it should be used in judicious amounts and again not in large thicknesses.

Materials commonly used for thermal mass

• Water. Water has the highest volumetric heat capacity of all commonly used material. Typically, it's placed in large container(s), acrylic tubes for example, in an area with direct sunlight. It may also be used to saturate other types material such as soil to increase heat capacity.

• Adobe brick or mudbrick. See Adobe.

• Earth, mud, and sod. Dirt's heat capacity depends on its density, moisture content, particle shape, temperature, and composition. Early settlers to Nebraska built houses with thick walls made of dirt and sod because wood, stone, and other building materials were scarce. The extreme thickness of the walls provided some insulation, but mainly served as thermal mass, absorbing heat during the day and releasing it during the night. Nowadays, people sometimes use earth sheltering around their homes for the same effect. In earth sheltering, the thermal mass comes not only from the walls of the building, but from the surrounding earth that is in physical contact with the building. This provides a fairly constant, moderating temperature that reduces heat flow through the adjacent wall.

• Rammed earth. Rammed earth provides excellent thermal mass because of its high density, and the high specific heat capacity of the soil used in its construction.

• Natural rocks and stones. See Stonemasonry.

• Concrete, clay bricks and other forms of masonry. The thermal conductivity of concrete depends on its composition and curing technique. Concretes with stones are more thermally conductive than concretes with ash, perlite, fibers, and other insulating aggregates.

• Insulating Concrete Forms are commonly used to provide thermal mass to building structures. Insulating Concrete Forms or ICF provide the specific heat capacity and mass of concrete. Thermal Inertia of the structure is very high because the mass is insulated on both sides.

Seasonal energy storage

If enough mass is used it can create a seasonal advantage. That is it can heat in the winter and cool in the summer. This is sometimes called "Passive annual heat storage or PAHS". The PAHS system has been successfully used at 7000 ft. in Colorado and in a number of homes in Montana.

See also

• Trombe wall
• Thermal energy storage
• Specific heat capacity

External links

• Thermal mass, a technical guide from the Australian Government.
• Thermal Mass - Energy Savings Potential in Residential Buildings
• Thermal conductivity and specific heat charts