It’s a common misconception that lighter building construction is better, in the sense of using lightweight concrete for floors, aluminum studs for walls, and intricate schemes to minimize the steel stock used. Structurally, this approach is valid. It reduces the weight on the structure and the static loading of the building skeleton. However, if energy conservation is the objective, it’s not always necessary to spend extra time and money on the exacting design of engineered building components, the purchasing of exotic materials, and the hiring of highly skilled craftspeople. In fact, it can be detrimental, if not to the expected lifetime of the building, then to its energy-use profile.
The key to understanding this seeming paradox is to think of building materials in terms of thermal mass. Imagine a wall facing south or west in the winter. If it’s constructed of inexpensive concrete or masonry, it will absorb the sun’s energy throughout the day, then radiate this heat inward after the sun sets to warm the inside spaces. During the summer this same wall delays the transmission of heat inside, making it easier for the HVAC equipment to cool the exterior zones (and often shifting these loads to off-peak hours).
In either case, the wall acts as a shock absorber on a car, consuming a portion of the thermal load, dissipating it, and smoothing out the thermal peaks. This is actually more than a literary analogy, but a mathematical one as well since the dynamic analysis of thermal mass is the same analytical function as that for a mechanical-vibration damper. The thermal mass of a building reduces both the heating and cooling peaks, so it affects the sizing of the air-conditioning equipment and the furnace.
Thermal mass lowers the peak load, i.e., the highest load that can be expected for a building during the year. It represents the maximum capacity required of the HVAC equipment during the heating and the cooling seasons. The peak load is the quantity used to size the air-conditioning equipment, so a lower peak translates into smaller equipment and lower first-time cost. It also means that the total annual utility bills are reduced.
HVAC equipment operates most efficiently when it’s operating at or near its capacity When a building has a high summer or winter spike in the load—the peak load at which the equipment is designed to operate, at maximum capacity—it runs all the rest of the time at a considerable amount below this peak. Lowering the peak by the use of thermal mass in the building reduces the variance between average and maximum loading, and the equipment is able to operate more efficiently. This analysis applies to both heating and cooling seasons, as the savings from thermal mass occurs every day.
Building elements other than the outside walls, such as interior furnishings and equipment in conditioned spaces, are also considered to be thermal mass. They help to lower the peak internal loads, which has a beneficial effect upon the equipment sizing and operation. This applies especially to exterior zones (e.g., those situated at or near the shell of the building), to zones with many windows, and to rooms with roof loads. All of these areas experience wide fluctuations in temperature, hence contribute more than interior zones to the peak building load.
There’s another advantage of thermal mass, for industrial users that pay a demand charge for electricity use. This peak-use fee is based on the highest rate of electricity use during any given month that occurs during the regular business hours. Most aspects of the building energy use are relatively consistent during this time, with the exception of HVAC power demand. It typically reaches a maximum late in the afternoon in the summer. A high building thermal mass reduces this maximum and consequently has a marked effect upon the electrical demand charge.
One final comment, on thermal mass as it applies to new construction or remodeling projects. Wall penetrations by windows and doors reduce the thermal mass and should be minimized. Daylighting from these offsets the detriment in thermal mass effects and is most effective as a proportion of wall space. For example, if a window area is so large that blinds are needed to shunt the natural light to comfortable levels in an office, the positive effects may be negated or even become an inconvenience. Also, if the window is so large as to make an office feel uncomfortable and disconnected from the adjacent work areas, the positive benefits of view upon the psyche are lost.
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Excerpt from Energy Conservation Projects.