Pore structure is an important factor that affects both vapor adsorption and water absorption performances of the expanded perlite insulation mortar. This study aims to explore the change in the pore structure of the expanded perlite insulation mortar in heat-rain cycles and investigate the effects on capillary water absorption and vapor adsorption performances. The expanded perlite insulation mortar specimens was subjected to 160 heat-rain cyclic test, and the distribution characteristics of the pore structures of the specimens after different heat–rain cycles in vacuum saturated water absorption, capillary water absorption, and vapor adsorption processes were measured via nuclear magnetic resonance. According to the test results, the pores with a size of greater than 1 μm in the expanded perlite insulation mortar determined the variation in total pores. The pore size can be classified into different ranges according to the performance in capillary water absorption and vapor adsorption. The calculated results of the fractional dimension suggest that the structure variation rules of different types of pores can be characterized by the fractional dimension. The relationship between the macro-performance parameters and the micro-structure can be established by using the fractional dimension as the linkage to provide reference for the hydrothermal evaluation of the whole expanded perlite external thermal insulation composite system. •The evolution of the pore structure of ETICS EP was measured by NMR.•The variation of adsorption and water absorption properties in aging process was analyzed.•The fractal dimension was used to characterize the pore structure of ETICS EP.