Facilitator of moisture accumulation in building envelopes and its influences on condensation and mould growth

•Seasonal moisture distributions in building envelopes with different orientations.•Surface insulation combined with high heat flow may motivate moisture to accumulate.•Elucidation of the process of moisture accumulation within the building envelopes.•Moisture accumulation induces fungi to perversely generate in building components.•Three favourable periods for condensation, spore germination, and mycelial growth. The hygric performance of building envelopes determines the degradation progress. Multiple factors, including the climate parameters (e.g., solar radiation and wind-driven rain) and building structure (e.g., thermal bridges), significantly affect the hygrothermal conditions in building envelopes. Inappropriate thermal insulation measures may exacerbate the humidity within the building envelope and further induce condensation and mould growth, which threatens the building structural safety and indoor air quality. However, the key facilitators of moisture accumulation in building envelopes are yet to be fully determined. Herein, a well-validated coupled heat and moisture transfer (HAMT) model was adopted to investigate moisture accumulation in building envelopes. The target building was located in a Chinese city (Hangzhou) where the climate is characterized as a hot and humid summer and a cold winter. The results demonstrate that the moisture content as well as condensation and mould risk is closely related to the building wall orientation. High heat flux at the exterior surface can reduce the overall moisture content of the building envelope. Conversely, it could facilitate moisture accumulation when the building envelope is exteriorly isolated by a layer of thermal insulation material (e.g., expanded polystyrene). Subsequently, the insulation layer could increase condensation risk. Three prone periods for condensation as well as spore germination and mould growth in a year were detected, indicating that the conditions that are tendentious for the above risks are the same. The adoption of an external insulation layer could prevent mould from growing inward while increasing the condensation risk.