Thermal Mass - Energy Savings Potential in Residential Buildings


The whole building energy calculation program DOE-2 E was utilized to simulate residential buildings containing simple multilayer wall assemblies. Simple walls without thermal bridges can be accurately represented by one-dimensional models like DOE-2 [Kossecka & Kosny - 1998, Kosny 2000, ASHRAE - 2001]. Four sets of massive walls representing different sequences of concrete and foam layers were simulated. Each set consisted of four walls of the same material sequence. These four wall sets (sixteen walls total) represented the majority of existing massive wall material configurations used in construction today. For all wall configurations analyzed in this section, the same material properties were used and are presented in Table 2.

The above walls had different thicknesses of concrete and insulation layers. For each analyzed material configuration, four different sets of thicknesses were considered are were organized according to their R-value;

- R - 3.03 m2K/W (17.2 hft2F/Btu), in total: 10.6 cm (4-in) of foam, 15.2-cm. (6-in) of concrete.

- R - 2.29 m2K/W (13.0 hft2F/Btu), in total: 7.6 cm (3-in) of foam, 10.2-cm. (4-in) of concrete.

- R - 1.58 m2K/W (9.0 hft2F/Btu), in total: 5.2 cm (2-in) of foam, 10.2-cm. (4-in) of concrete.

- R - 0.88 m2K/W (5.0 hft2F/Btu), in total: 2.5 cm (1-in) of foam, 10.2-cm. (4-in) of concrete.

Table 2. Thermal properties of material for multilayer walls.

Material Thermal conductivity W/mK (Btu-in./hft2F) Density kg/m3 (lb/ft3) Specific heat kJ/kgK ( Btu/lbF)
Concrete 1.44 (10.0) 2240 (140) 0.84 (0.20)
Insulating Foam 0.036 (0.25) 25.6 (1.6) 1.21 (0.29)
Gypsum Board 0.16 (1.11) 800 (50) 1.09 (0.26)
Stucco 0.72 (5.00) 1856 (116) 0.84 (0.20)

Due to the limited size of this paper, only some examples of the results are presented below. Detailed results for all considered houses are scheduled to be available at the end of 2001under the following Internet address:

Figure 3 depicts an example of the relationships between wall steady-state R-value and Dynamic R-value Equivalents for the Washington D.C. climate. A one-story ranch house of 143 m2 (1540-ft2 ) [Hasting 1977, Huang 1987] is chosen to illustrate the dynamic energy performance of a one-story residential building. Similar relations were observed for all considered climatic conditions and for all sizes and types of buildings. This data shows that the most effective wall assemblies were walls with thermal mass (concrete) being in good contact with the interior of the building (Intmass and CIC). Walls where the insulation material is concentrated on the interior side (Extmass) were the worst performing wall assemblies. Wall configurations with the concrete wall core and insulation placed on both sides of the wall (ICI) performed slightly better than Extmass configurations. However, their performance was significantly worse than CIC and Intmass configurations. The ICI configuration can be used for approximate analysis of the very popular Insulated Concrete Forms (ICFs) constructions, since ICF walls consist of the internal concrete core placed between shells made of insulating foam.

Figure 3. Dynamic R-value equivalents for Washington D.C. for 1540-ft2. one-story ranch house

The relationship between DBMS and wall R-value is not linear. For CIC and Extmass configurations DBMS is relatively close to 1.0. Figure 4 depicts DBMS values for a 143 m2 (1540-ft2) one-story residential building in the Washington D.C. climate. As in Figure 3, CIC and Intmass walls outperformed other wall systems. Walls where the insulation material is concentrated on the interior side of the wall have the smallest DBMS values. DBMS values for walls with the concrete core and insulation placed on both sides fell between these configurations. It was observed for all simulated cases that the DBMS was at its maximum for wall R-values between 2.3-3.0 m2K/W [13 - 17 hft2F/Btu].

Figure 4. DBMS values for Washington D.C. for 143 m2 (1540-ft2. ) one-story ranch house.

Figure 5 shows the relationship between wall material configurations and DBMS for ten climates. A one-story ranch house and two R- 3 m2K/W (17 hft2F/Btu) walls were considered. One wall had a concrete core with insulation placed on both sides and the second wall was built with concrete on the interior side and insulation on the exterior. The first wall exemplifies popular ICF systems used in the U.S. and Canada. The second wall could represent a concrete block wall insulated with external rigid foam sheathing. Figure 5 clearly demonstrates significant differences in energy performance between the two wall systems. The wall with external foam insulation (Intmass on Figure 5) was much more effective than the ICF wall. The most favorable climates for both wall systems were in Phoenix and Miami and the worst locations were Minneapolis and Chicago. However, even for the worst locations, the DBMS values were close to 1.5. The range of DBMS values for walls with exterior foam insulation (DBMS - from 1.4 to 2.8) is much wider than a very flat chart of DBMS values for the ICF wall system (oscillating around 1.5). This is caused by different distributions of mass and thermal insulation in these walls, generating significant differences in DBMS values for the same climate.

Figure 5. DBMS values for two massive wall systems in ten U.S. climates for 143 m2 (1540-ft2) one-story ranch house.

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Oak Ridge National Labs and Polish Academy of Sciences
Updated August 13, 2001 by Diane McKnight