Due to its density, concrete has the capacity to absorb and store large quantities of heat, contributing to a high-performance building envelope. Its thermal mass allows concrete to react very slowly to changes in outside temperature to reduce peak heating and cooling loads and delay the time at which these loads occur. The resulting savings can be significant—up to 25% of heating and cooling costs.
The following are excerpts from the PCI Designer's Notebook on energy conservation.
Thermal mass and energy savings
ASHRAE Standard 90.1 acknowledges the thermal mass benefits of concrete walls in specifying lower minimum insulation R-value and higher maximum wall U-factors for mass (concrete) wall construction.
Research conducted by Oak Ridge National Laboratory compared the dynamic thermal performance of insulated concrete walls with that of a traditional wood frame. Research shows that insulated concrete sandwich walls constructed with composite connector technology utilize the thermal mass effect of concrete to create an “equivalent wall performance R-value” several times greater than a traditional material R-value calculation.
Energy-saving benefits of thermal mass are most pronounced when the outside temperature fluctuates above and below the balance temperature of the building, causing a reversal of heat flow within the wall. The balance point is generally between 50 and 70°F.
These ideal conditions for thermal mass exist on a daily basis at all locations in the United States and Canada.
Another factor affecting the behavior of thermal mass is internal heat gain. This includes heat generated inside the building by lights, equipment, appliances and people; and heat from the sun entering through windows. Generally, during the heating season, benefits of thermal mass increase with the availability of internal heat gains. During the cooling season, thermal mass exposed to the building's occupied spaces will absorb internal gains, shifting peak cooling periods. Concrete exposed to the interior, not covered by insulation and gypsum wallboard, works best to absorb internal gains, saving cooling energy.
Thermal mass also works well when daily temperatures have large variations between the daytime high and nighttime low and when outdoor air can be used for nighttime ventilation. Designs employing thermal mass for energy conservation should be given a high priority.
Color (albedo) of precast concrete panels can be used to improve the energy-conserving features of the walls. Panels with high albedo (generally lighter in color) can help reduce the urban heat-island effect. Albedo is the ratio of the amount of solar radiation reflected from a material surface to the amount that shines on the surface.
Generally, materials that appear to be light colored have high albedo and those that appear dark colored have low albedo. On exterior surfaces, high albedo decreases solar heat gain; low albedo increases solar heat gain. A low albedo north wall and high albedo east and west walls and roof form the most energy-conserving arrangement in a northern hemisphere climate that uses both heating and cooling. High albedo surfaces are especially important where cooling dominates the energy requirements. It should be noted, however, that the color of the exterior walls has less effect on energy consumption when the walls have high R-values and thermal mass.
Light-colored exterior surfaces also help reduce urban heat-islands. Urban areas are up to 7°
F warmer than the surrounding areas. This difference is attributed to more buildings and pavements that have taken the place of vegetation. Where buildings and paved surfaces are required, using materials with higher albedos will reduce the heat-island effect, save energy by reducing the demand for air conditioning, and improve air quality.
Air infiltration has significant effects on the amount of energy required to heat and cool a building. Large precast concrete panels have minimal joints, reducing uncontrolled air infiltration.