Thermal Mass

The following text is the conclusion portion of a multiple page report conducted over many years of testing by the Oakridge National Laboratory on massive wall configurations for residential structures. The report explains that 16 different configurations of massive exterior walls were compared to each other as well as with wooden stud walls in 10 different climatic zones in the Continental United States.

The comparative study findings showed that massive exterior wall materials performed very well in certain climatic zones (like Bakersfield CA.) and that the benefits were not as substantial in other zones studied. The study also showed that in general, massive wall configurations for residential structures performed better than traditional wood frame structures in terms of energy efficiency and overall performance.

As you will read in the conclusion of the report, of the massive wall configurations tested, the most efficient configuration of massive walls is the configuration that has a foam core in the middle sandwiched with a concrete shell on both sides.

Review the full report published by Dr. Jan Kosny of the Oak Ridge National Laboratory: Thermal Mass – Energy Savings Potential in Residential Buildings.


Experimental and theoretical analysis of the energy performance of light-weight and massive wall systems was presented in this paper. Dynamic thermal performance of sixteen wall assemblies was investigated for residential buildings and the potential energy savings were presented for ten U.S. climates. It was found that some massive building envelope technologies can help in the reduction of building annual energies.

Several comparative field experiments have demonstrated that in many U. S. locations, heating and cooling energy demands in buildings containing massive walls of relatively high R-values can be lower than those in similar buildings constructed using equivalent R-value with lightweight wall technologies.

The thermal mass benefit is a function of wall material configuration, climate, building size, configuration, and orientation. From ten analyzed U.S. locations, the most beneficial for application of thermal mass are Phoenix, AZ and Bakersfield, CA.

Comparative analysis of sixteen different material configurations showed that the most effective wall assembly was the wall with thermal mass (concrete) applied in good contact with the interior of the building. Walls where the insulation material was concentrated on the interior side, performed much worse. Wall configurations with the concrete wall core and insulation placed on both sides of the wall performed slightly better, however, their performance was significantly worse than walls containing foam core and concrete shells on both sides.

Potential whole building energy savings, available when lightweight walls are replaced by massive walls of the same R-value, were calculated for 143 m2 (1540-ft2 ) one-story ranch houses located in Minneapolis, Minnesota and Bakersfield, California. For high R-value walls, up to 8% of the whole building energy could be saved in Minneapolis and 18% – in Bakersfield when wood-framed walls were replaced by massive wall systems. Thermal mass layers must be in good contact with the interior of the building in these walls.

Whole building possible energy savings in houses built with ICF walls were estimated as well. Three houses with 74 – 279 m2 (800-3000 ft2 ) of floor area were simulated for this purpose. It was found that for ten U.S. locations, ICF walls of R- 2.6 and 3.5 m2K/W (15 and 20 hft2F/Btu), the average potential whole building energy savings (ICF house vs conventional wood-framed house) can be between 6 and 8%.

by: Dr. Jan Kosny, Thomas W. Petrie, Phillip W. Childs, André O. Desjarlais, Jeffrey E. Christian

Oak Ridge National Laboratory