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L Table 1. Comparison of Approximate R-values and Framing Factors for Nominal 2 x 4 in.(5.1 x 10.2 cm)Wood Stud Walls In Series Only Studs Included Studs and Plates Included Fictitious Wall with 25%Framing Factor R-141 (Base Case) Stud Framing R-Value, Framing R-Value %Difference Framing R-Value %Difference Spacing Factor h•it2'FBtu Factor h'ft1°FBtu Factor h ft2°FBtu 16-in. 9.4% 12.7 14.1% 12.0 5.2% 25.0% 10.5 17.2% 24-in. 5.2% 13.3 11.0% 12.5 6.1% 25.0% 10.5 20.9% m2K/W maicw m2K/W 40-cm 9.4% 2.24 14.1% 2.11 5.2% 25.0% 1.85 17.2% 61-cm 5.2% 2.34 11.0% 2.20 6.1% 25.0% 1.85 20.9% R-vahie calculated in the center of wall cavity(without considering framing members)was R-14(2A6 or$/W) studs installed 16-in.(40-cm)o.c.The framing factors for all these wall assemblies were slightly greater than 24%.During these hot box tests,temperature differences across these test walls were between 40 to 45 °F(4.4 -7.2 °C)with the mean temperatures close to 75°F(23.8°C). As shown on Figure 2,the top and bottom plates,clusters of studs,and horizontal bracing were included in the test spec- imens.R-13 2.3(2.3 m2K/W)fiberglass baits were carefully <T' pJ cut to fill wall cavities without compression. Wall surfaces Wit were finished with %in. (1.3-cm) thick gypsum boards and OSB sheathing The first test wall was constructed with nomi- nal 2x4 in.(S.I x10.2-cm)wood studs.In the second and third walls standard C-shape 3.5-in.(5.1-cm)16-ga.light-gage steel framing was used. The third wall was similar to the second wall with the addition of-in.(1.9-cm)thick expanded poly- styrene foam sheathing on the exterior side of the steel studs. Test results are summarized in Table 2. Figure 2 Framing for wood stud wall assembly with 24% framing factor As shown in Table 2,in all tests,nominal center-of-cavity R-values were significantly larger than the test-generated clear-wall R-values. The first and second walls had the same center-of-cavity material It-values.The hot-box clear-wall R- value results, however, were 30% to 60% lower than the NUMERICAL THERMAL ANALYSIS center-of-cavity R-values. These measurements show that center-of-cavity R-values are a poor representation of the this work, various configurations of 2x4 wood and whale-wallthermal performance.In that light they shouldn't steel-framed walls were analyzed numerically for clear-wall R-values.The finite difference code,Heating 7.3,was utilized be directly used for code approvals, load calculations, or whole-building energy simulations. for this these calculations(Childs 1993).This code was cali- brated using a number of standard wood and steel-framed wall Many builders using light-gage steel framing believe that systems and its accuracy is well documented.(Kosny,Desjar- addition of -in. (1.9-cm) thick XPS foam sheathing will lais 1994, Kosny, Christian 1995B, Kosny, Childs 2002) In increase the clear wall R-value to the level of similar wood addition,the computer model was validated with the hot-box frame walls.This series of hot-box tests showed that the addi- test results for the steel stud wall that were part of this project. tion of a%-in.(1.9-cm)thick XPS foam sheathing to the 2x4 Three-dimensional computer simulations were within 5%of (5.1x10.2-cm) steel-frame wall doesn't contribute enough the thermal measurements. thermal resistance to match the thermal performance of the With the use of the calibrated computer model,each wall 2x4 in. (5.1x10.2-cm) wood-frame wall insulated with the configuration was analyzed for steady-state heat transfer in same type ofR-13(2.3 m2K/W)fiberglass ban insulation. three dimensions.Clear-wall R-values were calculated for the different wall configurations from calculated heat fluxes through the systems. 4 Buildings X