Thermal Mass - Energy Savings Potential in Residential Buildings


A wide selection of historic and current field experiments are discussed in the following section. Some early experiments were initiated in late 70's as a result of the energy crisis and focused on application of passive solar techniques in residential buildings.

Passive solar designers used glazing and thermal mass to utilize solar energy and stabilize interior air temperature. A Los Alamos National Laboratory team headed by J. D. Balcomb and R. D. McFarland investigated the energy performance of several passive solar wall systems and a various thermal mass storage materials. All systems were tested in field conditions in 2.6x1.9x2.9 m (100x80x120 in.) insulated lightweight containers [J. D. Balcomb et al. - 1978]. The only thermal mass provided was by the tested solar systems. Several materials were tested as a potential energy storage during these experiments. The most common was the application of conventional masonry blocks or solid concrete walls. However, Los Alamos researchers also studied the energy performance of water and phase change materials as energy storage means. The results from these experiments demonstrated that passive solar systems had a great potential in reducing energy consumption in residential buildings. They were published in the Passive Solar Handbook [J. D. Balcomb et al. - 1983] and Passive Solar Construction Handbook [Steven Winters Inc. - 1981] which have been widely used as a reference in the designing of passive solar houses.

Several other experiments focused on more conventional applications. These field studies demonstrated the potential energy demand reductions in buildings containing massive walls, floors, or roofs. It was observed and documented that heating and cooling energies in massive houses can be far lower than those in similar buildings constructed using lightweight wall technologies. This better performance resulted because the thermal mass encapsulated in the walls reduces temperature swings and absorbs energy surpluses both from solar gains and from heat produced by internal energy sources such as lighting, computers, and other appliances.

In June 1982, ORNL hosted the Building Thermal Mass Seminar [Courville, Bales 1982]. This seminar gathered a very interesting collection of results from theoretical and experimental studies on building thermal mass. Experimental work of T. Kusuda, D. Burch, and G.N. Walton from the National Institute of Standards (NIST), A.E. Fiorato from the Construction Technology Lab, and P.H. Shipp from Owens Corning, created a solid foundation for the future studies in this field. During the seminar, several presenters indicated a possibility of potential energy savings in houses using massive building envelope components.

Almost two decades ago, several thermal mass field experiments were carried out for DOE by researches in Gaithersburg, Maryland, Santa Fe New Mexico, and Oak Ridge, Tennessee [Burch 1984a, b, c, Robertson, Christian -1985, Christian 1983, 84, 85]. The primary focus of these projects was to collect reliable performance data for structures that emphasized exterior wall thermal mass effects. Several principal data-collecting efforts are described below.

Burch built four one-room test huts 6x6 m (20x20 ft) at the National Institute of Standards and Technology (NIST) to compare the seasonal energy performance of wood-framed, masonry, and log constructions. Site weather data were collected for periods during winter, spring and summer. The buildings were of identical construction except for the walls and were operated at the same thermostat setting. This study conclusively demonstrated the effect of thermal mass on space heating and cooling loads. Significant energy savings were noted for the house with a higher internal thermal mass.

During the same study, the impact of thermal mass on the night temperature setback savings was investigated. It was believed that night temperature setbacks might cause a significant reduction in the setback energy savings in massive buildings. The following observations were made during this project:

-When thermostat setpoint temperature was suddenly reduced by a fixed amount, the indoor temperature decreased from higher to lower level. During that period, the heating plant remained off. Thermal mass in buildings increased the time for the indoor temperature to decrease during the setback period.

- Similarly, during the morning period when the indoor temperature setpoint was increased, the presence of thermal mass extended the time to reach setpoint. The output capacity of the heating plant was sufficiently large that the temperature setup was short compared to setback.

The net effect of thermal mass in buildings containing heavyweight components was believed to cause the average indoor temperature and difference across the building envelope to be maintained at a more elevated level. As a result, night temperature setback caused the envelope heat-losses rate to be higher in massive buildings. All of this supported a common belief that night temperature setbacks in massive buildings caused a reduction in the setback energy savings. D. Burch investigated this penalty in setback energy savings and his research confirmed the fact that such a reduction took place. However, the magnitude of this phenomenon was very insignificant. For example, for a typical residence the difference in setback energy savings in the massive house and traditional wood-framed was predicted as only 0.3%.

Robertson and Christian investigated eight one-room test buildings 6x6 m (20x20 ft) that were constructed in the desert near Santa Fe, New Mexico, to determine the influence of thermal mass in exterior walls. The buildings were identical except for the walls (adobe, concrete masonry, wood-framed, and log). Data was collected for two heating seasons from mid-winter to late spring. This study demonstrated that on small windowless massive test huts, energy consumption can be up to 5% lower than in lightweight building. It is important to point out that during this study, the massive walls had about three-to-four times lower R-value than wood-framed walls (wood-framed wall R-value was about R-13 vs R-2 to R-5 for adobe, concrete masonry, and log walls). This gives completely different meaning to the 5% energy savings that were reported.

During three years of 1982 -84, Christian monitored an occupied 372 m2 (4000 ft2) dormitory constructed of massive building materials in Oak Ridge, Tennessee. This study demonstrated the potential for energy savings in buildings using massive envelope materials. Whole building energy simulations were performed employing the DOE-2.1B computer model. This computer model was calibrated using experimental data collected and analyzed during the testing period of the dormitory. Later, massive building envelope components in the computer model were replaced by wood-framed components. Predicted energy demands with the wood frame were compared with the energy required with the massive building components. Final comparisons showed a potential 10% savings in cooling energy and a 13% savings in heating energy.

In 1999 a field investigation on thermal mass effect in residential buildings was performed by the NAHB Research Center [NAHB RC-1999]. NAHB RC evaluated three side-by-side homes 102 m2 (1098 ft2) of floor area to compare the energy performance of Insulated Concrete Forms (ICF) wall systems versus traditional wood-framed construction. All three homes had identical orientation, window area, roof construction, footprint, duct-work, and air handler systems. This research provided another experimental evidence of the superior energy performance of buildings constructed using massive wall materials. A 20% difference was noticed between the ICF house and the conventional wood-framed house’s energy consumption. In the final report, NHAB researches concluded that this 20% difference was caused by the R-7 difference in wall R-values ( ICF wall R-value was about R-20, conventional 2x4 wood stud wall R-value was about R-13). However, simulation data developed by ORNL for a similar 121m2 (1300 ft2) one story house suggests that for the same climate a difference between R-20 and R-13 should yield a maximum 8 to 9% difference in annual whole building energy consumption. This suggests that most likely thermal mass related energy savings during the NAHB ICF study were in the neighborhood of 11%.

Currently, a field investigation of the effect of thermal mass in residential buildings is being performed by the Oak Ridge National Laboratory’s Buildings Technology Center with support from the Insulated Concrete Forms Association and the local Habitat for Humanity. The goal is to evaluate the relative energy performance of insulated concrete form (ICF) wall systems. A major task of the project is to field monitor the energy efficiency of a typical ICF residential building side-by-side with another house that has traditional 2x4 wood-framed walls installed on concrete masonry unit foundations (see Figure 1). The interior floor space and floor plan are identical as are the ceiling and floor construction, heating/cooling system, and ductwork for the single-story, 111m2 (1200 ft2) houses.

The field monitoring of the houses began in mid-June 2000 and will continue for a calendar year, during which time the houses will be unoccupied with the heating/cooling systems operated on identical schedules. This will allow a strong experimental basis for the differences in energy consumption due to the differing outside wall constructions.

The purpose of the monitoring for one year is to provide data sufficient to validate annual energy models of the two houses in the Knoxville climate. Developed computer models will be used to investigate benefits of the ICF construction in climates different from the field-test climate of East Tennessee. A detail report from this project will be available at the end of 2001.

Figure 1. ORNL/Habitat test houses.

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© Oak Ridge National Labs and Polish Academy of Sciences
Updated August 11, 2001 by Diane McKnight