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C304.3 Calculation Procedures and Assumptions
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The following procedures and assumptions shall be used for all Chapter 4 code calculations. R-values for air films, insulation and building materials shall be taken from Section C304.3.1 or C304.3.2, respectively. In addition, the appropriate assumptions listed, including framing factors, shall be used.
Exterior surfaces are areas exposed to the wind.
Semi-exterior surfaces are protected surfaces that face attics, crawl spaces and parking garages with natural or mechanical ventilation.
Interior surfaces are surfaces within enclosed spaces.
The R-value for cavity airspaces shall be taken from ASHRAE 90.1 Normative Appendix A. No credit shall be given for airspaces in cavities that contain any insulation or less than 0.5 inch (12.7 mm). The values for 3.5 inch-cavities (84 mm) shall be used for cavities of that width and greater.
The U-factors, C-factors, F-factors, and heat capacities for typical construction assemblies from ASHRAE 90.1 Normative Appendix A shall be used for all calculations unless otherwise allowed by applicant-determined assembly U-factors, C-factors, F-factors or heat capacities. Interpolation between values for rated R-values of insulation, including insulated, sheathing is allowed; extrapolation beyond values in the ASHRAE 90.1 Normative Appendix A tables is not.
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 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.
Insulation, including insulated sheathing, is not considered a building material.
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