Precast concrete can take the heat.
By Claude Goguen, P.E., LEED AP
Seven billion people on a planet with limited resources means that energy conservation is not optional. Our planet’s growing population creates an ever-increasing demand for limited power resources. In today’s world, every engineer and architect involved in the design of new structures is looking for ways to increase building heating and cooling efficiencies. And because buildings’ HVAC1 systems use about 30% of all U.S. energy, there is an increased demand for sustainable, energy-efficient building designs.
Newer and greener energy technologies like solar panels, wind turbines and geothermal systems are gaining in popularity, but one method of conserving energy has been around for a very long time: using a building envelope material with a high thermal mass. Energy demand is not just about using energy, it is about having a comfortable indoor temperature when and where we want it. Designs that take advantage of precast concrete’s exceptional thermal properties can provide a high level of service and occupant comfort with less energy input.
Precast is a heavyweight for power efficiency
What is thermal mass? It’s defined as the inherent property of a material to absorb heat energy. Precast concrete has high thermal mass, because a lot of heat energy is required to change its temperature. Lumber products are much easier to heat and therefore have lower thermal mass. High thermal mass materials, like precast concrete, act like thermal sponges, absorbing heat during the summer to keep the building’s interior cool, and storing heat from the sun to release it slowly at night when outdoor temperatures fall.
Precast concrete’s thermal mass serves to flatten daily interior temperature differentials and thereby reduce energy demands on a building’s HVAC system. The more we can minimize, or flatten, a building’s interior temperature differential, the more energy we conserve.
When outdoor temperatures are at their summertime peak, a precast concrete building’s interior remains cool, because it takes time for heat to penetrate concrete with its high thermal mass. This heat transfer results in a time lag shown in the accompanying diagram. As that heat is transferred, the temperature is also reduced, an effect referred to as “damping.” The naturally lower nighttime temperatures cool the precast concrete mass (building envelope) and allow the precast to absorb heat again the next day. Precast concrete’s high thermal mass effectively delays and minimizes the impact of outside temperature variations on the indoor climate. Consequently, this improves a structure’s energy efficiency, which is mandated by the national Energy Policy Act of 1992 for commercial buildings.
Measuring thermal mass
So how is thermal mass – the ability of a material to absorb, store and release heat energy – quantified? The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) 90.1-2010, “Energy Standard for Buildings Except Low-Rise Residential Buildings,” quantifies thermal mass effects based on a wall’s heat capacity. Heat capacity is defined as a wall’s weight per square foot multiplied by the specific heat of the material.
Specific heat describes a material’s ability to store heat energy, or the amount of heat energy (in Btu) required to raise the temperature of one pound of a material by one degree Fahrenheit. The specific heat of concrete and masonry is generally assumed to be 0.2 Btu/lb·F.
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. The coefficient of thermal conductivity, k, is a measure of the rate at which heat is conducted through a unit area of homogeneous material of unit thickness for a temperature difference of one degree.
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 buildings.
The thermal mass properties of precast concrete can also help projects obtain LEED credits. The LEED-NC2 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.
Precast concrete’s high thermal mass is but one of its many benefits; but it is an important asset for designers who are looking to cut energy costs associated with HVAC systems and still keep building occupants comfortable. For more information on the thermal mass of precast concrete, contact the NPCA technical staff at [email protected] or call
2 LEED-NC stands for Leadership in Energy and Environmental Design for New Construction.
Claude Goguen, P.E., LEED AP, is NPCA’s director of Technical Services.