Effect of relative humidity fluctuations on fungal growth kinetics of bio-based building materials

Bio-based building materials (wood and plant-based aggregates) are increasingly used for their low embodied carbon and favorable hygrothermal properties, yet moisture fluctuations can promote biodeterioration by filamentous fungi. This study quantifies the effect of controlled relative humidity (RH) fluctuations at room temperature on fungal growth kinetics on hemp, flax, straw and wood substrates. Specimens were conditioned in sealed chambers at RH setpoints of 85%, 94% and 97%, leading to alternating surface wetting/drying and transient moisture availability. Fungal development was assessed by combining (i) optical microscopy imaging (Keyence) for morphological characterization and quantification by imaging of surface colonization, and (ii) a standardized microbiological enumeration expressed as colony-forming units (CFU/g). This dual approach provides complementary insight into both colonized surface and viable propagules across material types. Results reveal a strong dependence of growth kinetics on surface moisture availability and material composition. For hemp aggregates, mean contamination reached 5e+04 CFU/g at 97% RH, whereas it dropped to 1.5e+02 CFU/g at 85% RH. Lower moisture availability slowed or partially inhibited growth; porous lignocellulosic substrates were more sensitive to hygrometric transients, while wood showed higher resistance under equivalent exposures. These findings emphasize transient surface moisture as a key driver of biodeterioration and support integrating humidity transients into durability assessments and drying strategies for bio-based building envelopes. Future work will couple RH cycling with temperature variations to better represent in-service conditions.