The focus of this paper is on the large-format wooden panel painting Maundy Thursday Altarpiece from Southern Germany. Its wooden support and paint layer were severely damaged due to high climatic fluctuations, above all dryness. The aim of the research project was to develop a low-risk, conservatively acceptable procedure for controlled in situ humidification. In an interdisciplinary approach, a practical monitoring concept on-site was linked to art technology analyses, surface monitoring, hygrothermal simulations, and climate chamber tests. Based on the results, an individual climate corridor for controlled humidification of the case study was developed with the help of an enclosure and implemented in two gradual moistening phases. The combination of conservative support, measurement technology, and digital assessment allowed a controlled approach to a conservation optimum without other active interventions in the original material. The results highlight the need for object-specific strategies and humidity corridors at the interface between conservation, climate adaptation, and sustainability. A deviation from museum standard recommendations (depending on the guidelines 40–60% rH) shows the special challenges of monument preservation.