Thermal mass
Encyclopedia
Thermal mass is a concept in building design which describes how the mass of the building provides "inertia" against temperature fluctuations, sometimes known as the thermal flywheel effect. For example, when outside temperatures are fluctuating throughout the day, a large thermal mass within the insulated portion of a house can serve to "flatten out" the daily temperature fluctuations, since the thermal mass will absorb thermal energy when the surroundings are higher in temperature than the mass, and give thermal energy back when the surroundings are cooler, without reaching thermal equilibrium
Thermal equilibrium
Thermal equilibrium is a theoretical physical concept, used especially in theoretical texts, that means that all temperatures of interest are unchanging in time and uniform in space...

. This is distinct from a material's insulative
Thermal insulation
Thermal insulation is the reduction of the effects of the various processes of heat transfer between objects in thermal contact or in range of radiative influence. Heat transfer is the transfer of thermal energy between objects of differing temperature...

value, which reduces a building's thermal conductivity
Thermal conductivity
In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....

, allowing it to be heated or cooled relatively separate from the outside, or even just retain the occupants' thermal energy longer.

Scientifically, thermal mass is equivalent to thermal capacitance or heat capacity, the ability of a body to store thermal energy
Thermal energy
Thermal energy is the part of the total internal energy of a thermodynamic system or sample of matter that results in the system's temperature....

. It is typically referred to by the symbol Cth and measured in units of J/°C or J/K (which are equivalent). Thermal mass may also be used for bodies of water, machines or machine parts, living things, or any other structure or body in engineering or biology. In those contexts, the term "heat capacity" is typically used instead.

## Background

The equation relating thermal energy to thermal mass is:
where Q is the thermal energy transferred, Cth is the thermal mass of the body, and ΔT is the change in temperature.

For example, if 250 J of heat energy is added to a copper gear with a thermal mass of 38.46 J/°C, its temperature will rise by 6.50 °C.
If the body consists of a homogeneous material with sufficiently known physical properties, the thermal mass is simply the mass of material present times the specific heat capacity of that 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 measured for the entire body in question, directly.

As an extensive property, heat capacity is characteristic of an object; its corresponding intensive property is specific heat capacity, expressed in terms of a measure of the amount of material such as mass or number of moles, which must be multiplied by similar units to give the heat capacity of the entire body of material. Thus the heat capacity can be equivalently calculated as the product of the mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

m of the body and the specific heat capacity c for the material, or the product of the number of mole
Mole (unit)
The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...

s of molecules present n and the molar specific heat capacity . For discussion of why the thermal energy storage abilities of pure substances vary, see factors that affect specific heat capacity.

For a body of uniform composition, can be approximated by
where is the mass of the body and is the isobaric specific heat capacity of the material averaged over temperature range in question. For bodies composed of numerous different materials, the thermal masses for the different components can just be added together.

## 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
HVAC
HVAC refers to technology of indoor or automotive environmental comfort. HVAC system design is a major subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer...

.

### Properties required for good thermal mass

Ideal materials for thermal mass are those materials that have:
• high specific heat capacity,
• high density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

Any solid, liquid, or gas that has mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

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 table of volumetric heat capacity for building materials is available here, 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 and 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. Thermal energy 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 thermal energy 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 be applied in a ratio from 6-8:1 for any area of north facing (Southern Hemisphere) or south facing (Northern Hemisphere) glazing above 7% of the total floor area. For example a 200 m2 house with 20 m2 of north facing glazing has 10% of glazing by total floor area; 6 m2 of that glazing will require additional thermal mass. Therefore, 36-48 m2 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
Adobe is a natural building material made from sand, clay, water, and some kind of fibrous or organic material , which the builders shape into bricks using frames and dry in the sun. Adobe buildings are similar to cob and mudbrick buildings. Adobe structures are extremely durable, and account for...

or rammed earth
Rammed earth
Rammed earth, also known as taipa , tapial , and pisé , is a technique for building walls using the raw materials of earth, chalk, lime and gravel. It is an ancient building method that has seen a revival in recent years as people seek more sustainable building materials and natural building methods...

houses. Its function is highly dependent on marked diurnal temperature variation
Diurnal temperature variation
Diurnal temperature variation is a meteorological term that relates to the variation in temperature that occurs from the highs of the day to the cool of nights.-Temperature lag:Temperature lag is an important factor in diurnal temperature variation...

s. The wall predominantly acts to retard heat transfer from the exterior to the interior during the day. The high volumetric heat capacity
Volumetric heat capacity
Volumetric heat capacity , also termed volume-specific heat capacity, describes the ability of a given volume of a substance to store internal energy while undergoing a given temperature change, but without undergoing a phase change...

and thickness prevents thermal energy from reaching the inner surface. When temperatures fall at night, the walls re-radiate the thermal energy back into the night sky. In this application it is important for such walls to be massive to prevent heat transfer into the interior.

#### Hot humid climates (e.g. sub-tropical and 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
Ventilation (architecture)
Ventilating is the process of "changing" or replacing air in any space to provide high indoor air quality...

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.

Cold incoming tap water may be piped through radiators to draw summer thermal energy from the air. In most areas, its initial temperature is 60 degrees. Since the existing plumbing is deep underground, it's well insulated from the heat of the day.

### Materials commonly used for thermal mass

• Water. Water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

has the highest volumetric heat capacity of all commonly used material. Typically, it is 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.

• Clay Brick, Adobe brick or mudbrick. See Brick
Brick
A brick is a block of ceramic material used in masonry construction, usually laid using various kinds of mortar. It has been regarded as one of the longest lasting and strongest building materials used throughout history.-History:...

Adobe is a natural building material made from sand, clay, water, and some kind of fibrous or organic material , which the builders shape into bricks using frames and dry in the sun. Adobe buildings are similar to cob and mudbrick buildings. Adobe structures are extremely durable, and account for...

.
• Earth, mud, and sod. Dirt's
Dirt
Dirt is unclean matter, especially when in contact with a person's clothes, skin or possessions when they are said to become dirty. Common types of dirt include:* dust — a general powder of organic or mineral matter...

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 thermal energy during the day and releasing it during the night. Nowadays, people sometimes use earth sheltering
Earth sheltering
Earth sheltering is the architectural practice of using earth against building walls for external thermal mass, to reduce heat loss, and to easily maintain a steady indoor air temperature...

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
Rammed earth
Rammed earth, also known as taipa , tapial , and pisé , is a technique for building walls using the raw materials of earth, chalk, lime and gravel. It is an ancient building method that has seen a revival in recent years as people seek more sustainable building materials and natural building methods...

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
Stonemasonry
The craft of stonemasonry has existed since the dawn of civilization - creating buildings, structures, and sculpture using stone from the earth. These materials have been used to construct many of the long-lasting, ancient monuments, artifacts, cathedrals, and cities in a wide variety of cultures...

.

• Concrete, clay bricks and other forms of masonry. The thermal conductivity
Thermal conductivity
In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....

of concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...

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.

• Log is used as a building material to create the exterior, and perhaps also the interior, walls of homes. Log homes differ from some other construction materials listed above because solid wood has both moderate R-value (insulation) and also significant thermal mass. In contrast, water, earth, rocks, and concrete all have low R-values.

### 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
Seasonal thermal store
A seasonal thermal store is a store designed to retain heat deposited during the hot summer months for use during colder winter weather...

or PAHS". The PAHS system has been successfully used at 7000 ft. in Colorado and in a number of homes in Montana.