Thermal mass (Building)

Thermal mass, in the most general sense, is any material that has the capacity to store heat. The following discussion pertains to its functional application in ecologically-sustainable building construction. When used correctly, it can significantly reduce the requirement for active heating and cooling systems and the consumption of active solar, renewable energy and especially fossil fuel technologies.

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Properties required for good thermal mass

Ideal materials for thermal mass are those materials that have:

• High specific heat,
• High density
• Low (but not extremely low) thermal conductivity (see insulation)

Such materials are able to slowly store or release (low thermal conductivity), relatively large quantities of heat per unit volume (high volumetric heat capacity) compared to other materials.

Thermal mass should not be confused with insulation. Materials used for insulation typically have much lower thermal conductivity than materials used for thermal mass and generally do not have a high capacity to store heat. They can reduce unwanted heat transfer but are not significant sources of heat in themselves. Often a combination of good insulation and thermal mass is used to achieve an optimum solution.

When correctly incorporated in building construction, it can be a useful method of controlling the flow or storage of heat to maintain thermal comfort.

The historic use of thermal mass in housing has been well established in hot arid climates or warm temperate regions. In cold temperate areas, it must be combined with good passive solar design to be effective. Its use in hot, humid areas is controversial.^[1]

Many building contain 'incidental' thermal mass e.g. timber frame, plasterboard ('drywall') and furniture. Specific materials which are used to increase thermal mass include clay brick, mud brick ('adobe'), stabilised or rammed earth (pise), stone, concrete and water.

Use of thermal mass in different climates

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

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 act 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

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. It 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 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 been devised to determine this.^[2] 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) 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 humid climates (e.g. sub-tropical/tropical)

The use of thermal mass is the most challenging in this environment where night temperatures remain elevated. It 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

• Adobe brick or mudbrick.

• Earth, mud, and sod. Dirt's thermal conductivity 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.

• Natural rocks and stones.

• Concrete 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.

• Water (often large tanks of water in an area with direct sunlight).

Where thermal mass is appropriate

One small-scale application of thermal mass finds common use in cool climates - a fireplace and chimney - but large-scale applications of thermal mass are usually only appropriate in hot climates such as the desert and the tropics. In temperate climates, large-scale applications of thermal mass can make a house difficult to heat and cool, especially when the thermal mass is a large berm of earth set against the house. In the winter, the thermal mass absorbs most of the heat from the furnace, preventing the furnace from heating the air effectively. In the summer, the thermal mass stores large quantities of heat from the outdoors. The air conditioner spends much of its energy cooling down the thermal mass, rather than cooling the air.

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.^{[citation needed]}

See also

• Biomass

References