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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.