Question-and-Answer Resource for the Building Energy Modeling Community
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Looks like different assumptions went into the assembly analysis. From the Title 24 Reference Appendices:
Assumptions: Values in this table were calculated using the zone calculation method. The construction assembly assumes an exterior air film of R-0.17, a 7/8 inch layer of stucco of R-0.18, building paper of R- 0.06 (BP01), continuous insulation (if any), the insulation / framing layer, 1/2 inch gypsum of R-0.45 gypsum board (GP01), and an interior air film 0.68. The steel framing is assumed to be 0.0747 inch thick with a 15 percent knock out. The framing factor is assumed to be 25 percent for 16 inch stud spacing and 22 percent for 24 inch spacing. The EZFrame internal default framing percentages are 15 percent for 16 inch stud spacing and 12 percent for 24 inch spacing. To account for the increased wall framing percentage the frame spacing input to the EZ Frame program is reduced to 13.218 inches for 16 inch stud spacing and 15.231 inches for 24 inch stud spacing. Actual cavity depth is 3.5 inch for 2x4, 5.5 inch for 2x6, 7.25 inch for 2x8, 9.25 inch for 2x10, and 11.25 inch for 2x12. High density R-30 insulation is assumed to be 8.5 inch thick batt and R-38 is assumed to be 10.5 inch thick. The thickness of the stucco is assumed to be reduced to 3/8 inch when continuous insulation is applied.
And from ASHRAE 2016 Appendix A:
For the purpose of Section A1.2, the base assembly is a wall where the insulation is installed within the cavity of the steel stud framing but where there is not a metal exterior surface-spanning member. The steel stud framing is a minimum uncoated thickness of 0.043 in. for 18 gage or 0.054 in. for 16 gage. The U-factors include R-0.17 for exterior air film, R-0.08 for stucco, R-0.56 for 0.625 in. gypsum board on the exterior, R0.56 for 0.625 in. gypsum board on the interior, and R-0.68 for interior vertical surfaces air film. The performance of the insulation/framing layer is calculated using the values in Table A9.2-2. Additional assemblies include continuous insulation uncompressed and uninterrupted by framing. U-factors are provided for the following configurations: a. Standard framing: Steel stud framing at 16 in. on center with cavities filled with 16 in. wide insulation for both 3.5 in. deep and 6.0 in. deep wall cavities. b. Advanced framing: Steel stud framing at 24 in. on center with cavities filled with 24 in. wide insulation for both 3.5 in.deep and 6.0 in. deep wall cavities.
I would review the assumptions above against your design assembly, and determine which is more appropriate. Or, perform the calculation for your assembly yourself (following ASHRAE section A9.4 or the Title 24 equivalent) to justify the value used to your code official/architect/engineer.
