When considering mold prevention strategies, the environmental conditions in which fungi grow need to be taken into consideration. This environment is often characterized by a time-dependent relative humidity, and porous substrate. Growth has mainly been investigated in steady-state experiments. Therefore, the goal of this study is to understand the hyphal growth of Penicillium rubens on porous gypsum, under dynamic humidity conditions. Spores of P. rubens were inoculated on porous gypsum containing nutrients, and placed in a small incubation chamber, allowing for microscopic hyphal observation. The relative humidity in this chamber varied multiple times between a high (close to 100%) and low value (35%, 55%, or 75%). The hyphae reacted to a lowered relative humidity by an immediate growth stop and dehydration. When the relative humidity was increased again, the hyphae re-hydrated and three responses were found: regrowing after approximately 4 h, after a time equal to the germination time, or no regrowth at all. No substantial regrowth was found for fluctuations faster than 4 h. This time-scale was found for multiple decreases in relative humidity, and has been reported for the first time.