Abstract: The incorporation of phase change materials (PCMs) in building envelopes for passive thermal storage can enhance the thermal mass effect and thereby reduce energy consumption. In this investigation, multi-scale analysis of cementitious mortar and concrete containing microencapsulated PCM (MPCM) was performed experimentally and using numerical simulations. A three-dimensional two phase random composite model, which can be integrated with finite element method, was developed to predict the effective thermal properties of cementitious mortar and concrete with MPCM. MPCM was considered as inclusions in a continuous mortar matrix and the latent heat of PCM was incorporated into the simulations. The results showed that the effective thermal conductivity is strongly correlated with the volume fraction of PCM and is independent of the spatial distribution of the inclusions. These predictions were within the upper and lower bounds of parallel and series analytical models and agreed well with the experimental data (correlation coefficient 0.96 for concrete and 0.98 for mortar). Numerical simulations of the macro-scale behaviour of mortar and concrete with PCM for passive thermal storage showed a reduction in the maximum heat flux and time lag effect subjected to diurnal temperature variations. However, an optimum amount of PCM should be selected to fully exploit these passive systems. The developed models can be applied for optimising the design of composites to achieve the best thermal performance.