•A multiscale method for cementitious composite thermal conductivity is proposed.•Thermal cracks caused by high-temperature load generate crack thermal resistances.•Crack thermal resistances induce significant reduction in thermal conductivity.•Interfacial thermal resistance (ITR) effect dominates crack thermal resistance effect.•Synergistic effect of heat bridge and ITR is a main mechanism for conductivity reduction. The mesoscopic thermal cracking behavior in steel fiber reinforced concrete (SFRC) caused by high-temperature load can result in a substantial reduction of effective thermal conductivity (ETC) at macroscale level. In this study, a multiscale homogenization method for identifying the ETC of thermally damaged SFRC is proposed. This new method is characterized by the consideration of crack thermal resistance effect rather than using temperature-dependent thermal conductivities of components in cementitious composite. A series of experiments on thermal conductivity of mortar, plain concrete, and SFRC heated at various temperatures is performed. The method predictions satisfy the experimental results extremely well when the interfacial thermal resistance (ITR) coefficient is assumed to increase linearly with temperature. The numerical and experimental results show that the ITR effect makes approximately 50%, 36%, and 7%–12% contributions to the overall thermal conductivity reductions in the mortar, plain concrete, and SFRC heated up to 600 ∘C, respectively. Moreover, it is found that the trend and magnitude of ETC reduction caused by heating increase with the increase in particle content and particle-matrix thermal conductivity ratio due to the synergistic effect of heat bridge and ITR.