Study on water vapour transport performance of expanded perlite insulation mortar under hygrothermal coupling

Abstract The water vapour transport performance of expanded perlite insulation mortar (EPIM) is highly susceptible to hygrothermal aging, yet the underlying transport mechanisms under different humidity conditions remain inadequately understood. This study systematically investigated the water vapour permeability of EPIM subjected to artificial accelerated heat-rain cycles (0–80 cycles). The variations in mass, water vapour permeance, and equilibrium moisture adsorption were analyzed under two relative humidity differences (ΔRH = 25% and 48%). The results demonstrate that aging through heat-rain cycles substantially alters the moisture transport mechanism by modifying the material’s pore structure. Under low humidity gradients, water vapour transport is primarily governed by vapour diffusion, with permeance initially decreasing and then increasing as heat-rain progress, indicating high sensitivity to pore structure changes. In contrast, under high humidity gradients, capillary condensation triggers liquid water transport as the dominant mechanism, and permeance exhibits minimal variation with heat-rain cycles, suggesting a greater reliance on macro-pores. Furthermore, the study reveals that moisture adsorption behaviour is influenced by both the extent of aging and environmental humidity. In high-humidity environments, pronounced capillary condensation effects can overshadow the impact of certain pore structure changes. This research provides experimental evidence to elucidate the moisture transport mechanism of EPIM under hygrothermal environments and offers valuable insights for its durability design and performance prediction in building envelopes.