Thermal Mass

"Thermal mass" is a term used to describe the ability of building materials to store heat, or their thermal storage capacity. The basic characteristic of thermal mass is their ability to absorb heat, store it, and release it at a later time. A very common example of thermal mass in a home is a concrete slab.

During the day, the sun's light will warm a house and "recharge" any thermal mass the house may have. For example, a concrete slab will absorb some of the heat that results from the sun shining on the house. At night, when there is no sunlight to heat the house, the heat that was absorbed during the day will be gradually released. By morning the concrete has cooled and is ready to absorb heat during the day, and the cycle repeats.

The use of thermal mass does not increase or decrease total energy available, nor does it change the long-term heat loss or gain of the building. However, thermal mass does help to reduce energy demands in a home under mild heating and cooling climate conditions. It also helps reduce extreme temperatures within the home, making a more moderate temperature inside the home year round. The use of heavy weight construction materials with high thermal mass, including concrete slabs and insulated walls, can reduce total heating a cooling loads by 25%.

Heat flows from warm objects to cool objects by way of conduction, convection, and radiation. Conduction is transfer of heat through physical contact, such as a thermometer under your tongue. Convection is transfer of heat through a fluid, such as an ice cube cooling a glass of water. Radiation is the transfer of heat through free space, such as warming your hands on an open fire, or feeling the heat from the sun's rays.

Thermal masses use these three types of heat transfer in a four-step process:

Heat is radiated through the surface of the mass by a warmer object (such as sun, lights, people, or equipment).
Heat is conducted from the warmer surface of the mass to the cooler interior of the mass, effectively "storing" heat in the mass.
When the mass surface becomes warmer than other objects surrounding it, the mass radiates heat to these objects (meaning the mass radiates heat back into the house).
Heat from the warmer interior of the mass is conducted to the surface of the mass as the mass cools (a reversal of step 2).

For thermal mass to be effective, this process must occur in a time frame that approximates the thermal cycle of the building, usually daily.

Effective thermal masses have several key characteristics:

High heat capacity (the ability to store large amounts of heat)
Moderate conductance (must be able to transfer heat fairly well through conduction)
Moderate density (cannot be too heavy or too light)
High emissivity (must be able to easily emit, or give off, heat)

Volume and thickness of the thermal mass determines how much the interior temperature of the house will vary. Typical concrete slabs in homes are several inches thick, and are usually found under rooms of the home at ground level or below.

Maryland's 2005 Solar Decathlon house features a slab of lightweight gypsum. This material has most of the thermal mass characteristics of concrete, but does not weigh as much as concrete, making our house easier to transport. In addition, gypsum is easier to pour, and is self leveling, making installation simpler than concrete. For more information on pourable, lightweight gypsum flooring, please visit www.gypsumsolutions.com.

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