Title 24 vs 90.1 U-factors for metal frame walls

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ASHRAE 90.1 uses the series/parallel calculation method to determine the insulation value of wall assemblies which takes the weighted area average of heat flux through the frame and insulation sections. This method works fine for most wall assemblies but in regards to calculating walls with steel framing, this is not the recommended way. This is because the metal frame has a significantly different thermal resistance compared the cavity insulation so the frame section will also reduce the effectiveness of the cavity insulation touching the frame.

The California Energy Commission (CEC) funded their own research that lead to the development of Joint Appendix 4. I unfortunately couldn't find the results of this research but from what I've heard, they did actual testing of various wall assemblies as well as THERM models. Based the research, JA4 uses the modified zone method of calculation which divides the insulation section to multiple sections: the insulation in between the frames and insulation adjacent to metal frame. Otherwise, the calculation is similar to the parallel path method, just with more parallel paths. This same method is used to calculate the U-value of windows which is why NFRC rated windows have a assembly U-value and a center of glass U-value.

ASHRAE needs to update Appendix A since it is using an inappropriate calculation method for metal framed constructions. Title 24 JA4 is much more realistic in terms of actual performance of the wall, so it is recommended to use those values instead of the ASHRAE values.

If you want to learn more about the differences in parallel path method and zone method, I'd recommend reading about it in the ASHRAE Handbook of Fundamentals Chapter 27 which goes over the different methods as well as why parallel path method is wrong for metal frames.

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.