C304.1 Building Material Thermal Properties, General
R-values for building materials used to demonstrate code compliance with Chapter C4 shall be taken from ASHRAE 90.1 Normative Appendix A, from manufacturer's product literature or from other nationally recognized engineering sources. Assembly U-factor calculations shall follow the procedure(s) detailed in Section C304.3 or be tested in accordance with procedure(s) described in Section C304.2.
Concrete block R-values shall be calculated using the isothermal planes method or a two-dimensional calculation program, thermal conductivities from ASHRAE 90.1 Normative Appendix A and dimensions from ASTM C90. The parallel path calculation method is not acceptable.
Exception: R-values for building materials or thermal conductivities determined from testing in accordance with Section C304.2.