Thermal mass
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Thermal mass, in the most general sense, is any mass that absorbs and holds heat. In the architectural sense, it is any mass that absorbs and stores heat during sunny periods when the heat is not desirable in the living space of a building, and then releases the heat during overcast periods or during the night, when the heat is desirable.
Thermal mass normally involves some type of solar heating:
- For example, in passive solar architecture, the building might use a sun-facing, uninsulated wall made of adobe, stone, or concrete, called a trombe wall, to directly absorb sunlight. In some buildings, this wall is on the interior, and glass installed in front of the wall increases heat collection by reducing convective heat loss and by focusing the sunlight.
- In active solar architecture, the building transfers heat between the collectors, the thermal mass, and the living space using water or air, often with a complex pump system.
Properties required for a 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,
Think of specific heat as a material's "reluctance" to change temperature. More precisely, specific heat is a material's ability to store large quantities of heat per unit mass and per degree of temperature change.
Materials that are good for thermal mass are able to slowly store, and slowly release, relatively large quantities of heat per unit mass compared to other materials.
Air would be a candidate for use as a thermal mass, were it not for convection currents that quickly disperse its heat. Trapping air in some kind of substate, such as polystyrene, nearly stops convection, but reduces thermal conductivity into a range that is inappropriate for thermal mass applications.
Thermal mass is distinct from insulation. The confusion results because people think that the thermal mass is "insulating" the building, when the thermal mass is primarily smoothing out temperature swings in the interior of the building, and only insulating to a small degree. Materials used for insulation typically have much lower thermal conductivity than materials used for thermal mass. A material with high thermal conductivity will not work well as thermal mass because it will release its stored heat too quickly. Conversely, a material with extremely low thermal conductivity (around the thermal conductivity of materials used for insulation) will take too long to absorb and store heat.
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 Southwest 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.Caveats
The wall used for thermal mass should have the appropriate thickness, so that the interior of the building remains cool during the day, and heat is transferred into the interior during the night. If the wall is too thin, the heat will penetrate into the living space during the day when it is not needed, and there will not be enough stored heat left over to keep the living space warm in the evening. On the other hand, if the wall is too thick, the heat will take too long to penetrate the wall, and when it does finally reach the living space, it may release heat when you don’t want the extra heat.External links
- [Thermal Mass - Energy Savings Potential in Residential Buildings]
- [Thermal conductivitiy and specific heat charts]
- [Insulation and Weatherization Handbook (tabbed Excel version)]
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