Thermal Mass

By Claude Goguen, P.E., LEED AP

Thermal mass is used in the building industry to define the inherent property of a material to absorb heat energy; it has nothing to do with insulating materials. A wall material with high thermal mass can moderate daily temperature variations.

A lot of heat energy is required to change the temperature of high-density materials like precast concrete, and this is why concrete has high thermal mass. Wooden walls, by comparison, are much easier to heat with their lower thermal mass. High thermal mass precast concrete walls act like thermal sponges, absorbing heat during the day, and then slowly releasing the heat as temperatures fall at night.

Precast improves energy performance
Precast concrete’s thermal mass flattens out daily temperature differentials and therefore reduces heating and cooling loads on the building’s HVAC system. The resulting energy savings increase with high swings in ambient temperature. When outdoor temperatures are at their peak, the inside of the building remains cool, because the heat has not yet penetrated the precast concrete mass, producing a time lag as seen in Figure 1. Precast slowly releases heat into the interior spaces during late afternoon and evening when buildings are usually empty. This delay in heat transfer is known as “damping.”

Precast concrete’s mass is cooled by natural ventilation during the night, and then it is able to absorb heat again the next day. The result of this delaying or damping effect is improved energy conservation, which is mandated by the national Energy Policy Act of 1992 for commercial buildings.

So how does one quantify thermal mass? Specific heat describes a material’s ability to store heat energy. Specific heat is defined as the ratio of the amount of heat required to raise the temperature of a unit mass of a material by one unit of temperature to the heat needed to raise an equal mass of water by the same unit of temperature.¹

Thermal mass is not the same as R-value
Thermal mass is not to be confused with R-value, also known as thermal resistance. R-value is expressed as the thickness of the material divided by the thermal conductivity.²  R-values and U factors (thermal transmittance) do not take into account the effects of thermal mass, and by themselves are inadequate in describing the heat transfer properties of construction assemblies with significant amounts of thermal mass such as precast concrete.

The thermal mass properties of precast concrete can also help obtain LEED credits. The LEED-NC Energy & Atmosphere (EA) Credit 1 on optimizing energy performance can potentially provide up to 10 points for energy cost savings beyond ASHRAE Standard 90.1-2010.

Improved energy performance is just one of many benefits of precast concrete, but it is an important asset for designers who need to cut heating and air conditioning costs associated with HVAC systems and still keep building occupants comfortable.

For more information on thermal mass of precast concrete, contact NPCA’s Technical Services staff at [email protected] or at (800) 366-7731.

Claude Goguen, P.E., LEED AP, is NPCA’s director of Technical Services.

¹ The specific heat of concrete and masonry can generally be assumed to be 0.2 Btu/lb F. (ASHRAE Handbook of Fundamentals).
² The coefficient of thermal conductivity is a measure of the rate at which heat (energy) passes perpendicularly through a unit area of homogeneous material of unit thickness for a temperature difference of one degree.