KIEAE Journal

IECC uses the 17 zone classification scheme which was developed by Briggs et al. [10] using temperature, radiation, wind and humidity (Figure 1, left). Climate zones are numbered from 1 to 8, with higher zone numbers representing colder climates, and are further divided into moist (A), dry (B), and marine (C) regions. State of Illinois belongs to Climate Zone 4A and 5A.

Figure 1. 
2015 IECC Climate Zones (left) and Illinois Climate Zones with TMY3 weather files for simulations (right)[10]

In order to investigate CMU mass wall effects occurred by different weather in Illinois State, three TMY3 weather files were selected based on the geographical location; 1) Northern Region: Chicago TMY3 weather file (Climate Zone 5A), 2) Central Region: Springfield TMY3 weather file (Climate Zone 5A), and 3) Southern Region: Carbondale TMY3 weather file (Climate Zone 4A) (Figure 1, right). The summary of the climate characteristics is shown in Table 2.

According to U.S. Census Bureau [8], the median size of a completed single-family house in 2016 was 2,422 ft² (225 m²). In order to simplify the base-case simulation model, 2,500ft² (232 m²) single-family detached residence with a square-shape plan was considered as the basis for the simulations. Building envelope and system properties such as R-value, SHGC and HVAC equipment efficiencies were modeled as specified by the 2015 IECC [6]. Other building characteristics which were not specified by the 2015 IECC were modeled based on the Building America Housing Simulation Protocols developed by National Renewable Energy Laboratory [9]. The Building America Housing Simulation Protocols (HSP) document was sponsored by the U.S. Department of Energy (DOE), and it provides guidance to the simulation model.

The base-case model for the simulations is a single story residence with 9 ft (2.7m) wall height, slab-on-grade and an unconditioned- vented attic. Base-case model has a 15% window-to wall ratio and fraction of this window area has been equally distributed on each façade (i.e. 25% window area on each front, back, left and right facade). For consistency of the simulations, the same building and system configurations were assumed for all buildings and locations conforming to the 2015 IECC [6] and NREL [9] while changing only the wall type. Table 3 shows the model specifications of simulations except wall types. Wall types have been explained in Table 4.

In order to investigate the mass wall effectiveness, CMU mass walls were compared with typical wood frame walls. Typical wood frame walls consist of vinyl siding, OSB (Oriented Strand Board), insulation, stud and gypsum board. As specified in the 2015 IECC (Table 1), two wood frame walls were modeled; 1) R-20 (3.52 m²⦁°C/w) with 2“x6” (5cmx10cm) @ 24” (61cm) O.C. (On Center) stud (Figure 2, left), 2) R-5 XPS (0.88 m²⦁°C/w) as continuous insulation and R-13 (2.29 m²⦁°C/w) with 2x4 @ 16” (41cm) O.C stud (Figure2, right).

Figure 2. 
Wood frame wall wth R-20 insulation (left) and R-13+5 insulation (right) as specified in the IECC 2015

In order to find the general CMU wall types for a residence, CMU wall configurations from Concrete Masonry Association [11] were referred. According to Concrete Masonry Association, there are three CMU wall types; interior insulation, integral, and exterior insulation. The interior insulation is used to separate the interior from the mass wall as decreasing the ability of the mass walls in maintaining room temperature (Figure 3, top, left). Integral insulation is a method of filling insulation materials such as perlite in the cell of CMU (Figure 3, top, right), and exterior insulation is that insulation is installed outside of CMU wall as the mass wall is exposed to inside (bottom). It has been known that exterior insulation is the least affected by the outdoor conditions among the three insulation strategies. Since the 2015 IECC did not specify the integral insulation, CMU wall with integral insulation does not comply with the IECC. However, the study included this wall type for comparison because integral insulation is also one of the popular insulation strategies in the industry. In addition, three different sizes of CMU (6” (15cm), 8”(20cm) and 12” (30cm)) were used for the analysis to investigate the effect of CMU wall thickness.

Figure 3. 
Three CMU wall insulation strategies: interior (top, left), integral (top, right), and exterior (bottom).

Eleven wall types with different insulation level were modeled for comparison as shown in Table 4. Wall type #1 and # 2 indicate the typical wood frame wall, wall type #3 to #5 indicate CMU walls with exterior insulation, wall type #6 to #7 indicate the integral insulation and wall type #9 to #11 indicate CMU walls with interior insulation. Each wall type shows each layer from exterior to interior. R-value of each wall was selected to comply with the 2015 IECC (Table 1) except the wall type #6, 7, and 8 (integral insulation strategy) because insulations were not installed on these wall types.

The electricity and gas consumption of the simulation results are converted to energy costs using the most recent state-specific residential fuel price of DOE's energy information management [12 and 13]. Fuel price of electricity was $0.121/kWh and gas price was $0.788/therm plus $8.00 monthly service charge.

Carbondale TMY3 weather file was used for the simulations of the southern region. The analysis showed that the cooling energy use of wall types #1 and #2 (wood frame walls) was the same at 2116.1 kWh/year, and heating energy use was 58.4 MBtu/year (17,111 kWh/year) and 58.3 MBtu/year (17,088 kWh/year) respectively. This means that R-20 (3.52 m²⦁°C/w) cavity insulation has very similar thermal performance with R-5 (0.88 m²⦁°C/w) continuous and R-13 (2.29 m²⦁°C/w) cavity insulation. After changing the wall type from wood frame walls to CMU mass walls, cooling energy increased by 10.9% on average. Heating energy use increased by 2.7% for exterior insulation, and decreased by 1.6% for interior insulation. When comparing the results from exterior insulation and interior insulation of the same thickness of CMU wall (i.e. wall type #3 vs. #9, #4 vs. #10, and #5 vs. #11), interior insulations showed less cooling and heating energy consumptions. It is because the requirement of interior insulation (R-13) (2.29 m²⦁°C/w) was higher than exterior (R-8) (1.41 m²⦁°C/w) on the 2015 IECC even though the exterior insulation has been known to be better efficient than interior insulation. As expected, the integral CMU walls (wall types #6, #7 and #8), which were not compatible with the 2015 IECC, showed higher cooling and heating energy use than the other wall types. For annualized utility bills, #1 and #2 wood frame wall types showed the lowest utility bills ($1875.8 (₩2,065,000) and $1876.5 (₩2,066,000)respectively) than the other wall types. Among the CMU walls, #5 and #11 wall types, which used 12” (30cm) CMU, showed the lowest utility bills than the other CMU walls.

Springfield TMY3 weather file was used for the simulations of the central region. Cooling energy use decreased and heating energy use increased compared to the southern region. These variations in the cooling and heating energy use simply reflected the colder climate condition of this location. The electrical energy use of wall types #1 and #2 (wood frame wall) showed the same at 1623.7 kWh/year, which decreased by 23.3% from the southern region. Heating energy use was 80.1 MBtu/year (23,481 kWh/year), which increased by 37.2% from the southern region. The wood frame wall types showed lower cooling energy use than the CMU wall types, while CMU wall types with insulations showed lower heating energy use than the wood frame wall. Cooling energy use increased by 5.6% average and heating energy use decreased by 4.0% after changing wall types from the wood frame walls to CMU mass walls.

The comparison of annual cooling energy use of CMU wall types showed that exterior insulation strategy (wall types #3, #4 & #5) reduced cooling energy slightly than the interior insulation strategy (wall types #9, #10 & #11). Exterior insulation strategy resulted in slight increase of heating energy than the interior insulation strategy.

For annualized utility bills, #11 and #5 wall types using 12” (30cm) CMU showed the lowest utility bills than the other wall types because of heating energy reduction. A large variation in the heating energy use for space heating was observed in this location than the southern region. It indicated higher savings in heating energy use due to the thermal mass properties of CMU walls. The integral CMU walls (wall types #6, #7 and #8), which were not compatible with the 2015 IECC, showed the most electrical and thermal energy use.

Chicago TMY3 weather file was used for the simulations of the northern region. The patterns of simulation results from the northern region were similar to that of the central region as cooling energy use decreased and heating energy increased. When wall types were changed from wood frame walls to CMU mass walls, cooling energy use increased and the heating energy use decreased. Cooling energy use and heating energy use of the wood frame walls (wall type #1 and #2) showed 1304.2 kWh/year (38.4% lower compared to the southern region) and 97.5 MBtu/year (28,556 kWh/year) and 97.4 MBtu/year (28,529 kWh/year) (67.0% higher compared to the southern region) respectively. It was also found that the wood frame walls (#1 and #2) showed less cooling energy use (4.5% average difference) and higher heating energy use (3.5% average difference) than the CMU walls.

For CMU wall comparisons of the same thickness, exterior insulation strategy (wall type #3, #4 and #5) provided more benefit in saving cooling energy use, and interior insulation strategy (wall type #9, #10 and #11) yields better contribution to saving heating energy use. For annual utility bills, wall types #11 and #5 showed the lowest utility bills over the year.