The airspaces existing in exterior walls and window frames influence the overall heat transfer of these building components. The heat transfer in the airspaces consists of convective heat transfer and radiative heat transfer. The radiative heat transfer coefficient of the airspaces varies largely with their surface emissivities, surface temperatures, and two- or three-dimensional (3-D) shapes. Therefore, it is important to calculate the radiative heat transfer coefficient of the airspaces as accurate as possible considering the effects of these factors. In this paper, the calculation equations of the 2-D radiative heat transfer coefficient adopted in ISO 6946, ISO 10077-2, ISO/FDIS 15099, and JIS A 2101 are derived and the necessary assumptions for the derivations are summarized. Then it is shown that the 3-D equation is easily derived from the 2-D equation adopted in ISO/FDIS 15099. Numerical calculation method of 3-D equation considering the effect of the surface temperature distribution is developed. The recommended ranges for the use of the equations are summarized on the basis of the comparison with the accurate numerical method.
