C304.3.2.2 Applicant-Determined Assembly U-Factors, C-Factors, F-Factors, or Heat Capacities
If the building official determines that the proposed construction assembly is not adequately represented in the appropriate table of ASHRAE 90.1 Normative Appendix A, the applicant shall determine appropriate values for the assembly using the assumptions in ASHRAE 90.1 Normative Appendix A. An assembly is deemed to be adequately represented if:
- the interior structure, hereafter referred to as the base assembly, for the class of construction is the same as described in ASHRAE 90.1 Normative Appendix A and
- changes in exterior or interior surface building materials added to the base assembly do not increase or decrease the R-value by more than 2 from that indicated in the descriptions in ASHRAE 90.1 Normative Appendix A.