Mikkola and Wichman solve the differential form of the heat transfer equation in order to provide a theoretical foundation to their correlations of experimental data. Using the critical temperature criterion, they solve the one-dimensional conduction equation with a linearized radiative and convective boundary condition. The thermally thick (e.g., wood siding) and thermally thin (e.g., paper) limits are derived along with a solution for the intermediate case. The solutions for the thin and thick cases are discussed below. Although the thermally intermediate solutions are available, data are generally not correlated using the thermally intermediate results.