Wood-brick heritage buildings, iconic cultural structures across East Asia with timber frames and masonry foundations, are increasingly subject to adaptive reuse. However, the impacts of different usage patterns on their hygrothermal environments remain insufficiently understood. This study investigated two heritage buildings in the Forbidden City in Beijing: one adaptively reused as an office and the other unoccupied. Through a year-long monitoring of indoor temperatures, relative humidity, and surface temperatures, the study analyzed the deterioration risks of condensation and mold growth, as well as human thermal comfort, to optimize environmental regulation strategies. Results reveal that office-related activities significantly altered the indoor hygrothermal environments. The heating and cooling systems narrowed the annual indoor temperature ranges by about 21 ℃ and expanded relative humidity ranges by about 18%. This alteration improved thermal comfort for about 80% of the winter and summer hours, and mitigated microbial initiation risks in transitional seasons. Nevertheless, the intermittent AC operation increased daily temperature fluctuations by 1.3 to 2.3 times, enhancing the risks of condensation on floors and wall bottoms during the cooling period and thermal stress between interior and exterior wall surfaces. To ensure sustainable conservation and utilization of heritage buildings, this study recommends elevating summer AC set-points by 2–3 ℃, adding climate buffers at entrances, and equipping unoccupied buildings with environmental sensors to guide transitional-season ventilation decisions. These data-driven strategies reconcile preservation needs with adaptive reuse, providing transferable solutions for wood-brick heritage buildings.