The combination of the monumental building, the Rotonde, and the panoramic painting ‘Panorama Mesdag’ is one of the few remaining exhibitions of its kind. The organization of the museum aims to preserve the building and painting while also wanting to decrease the carbon footprint of the museum. The objective of this research is to design a sustainable climate solution using thermal buffering that creates an indoor climate that preserves the panoramic painting and reduces energy consumption of the minimally insulated building. The potential thermal buffering solutions were defined through literature research and the option of a PCM system secured to the interior façade was further elaborated on by estimating its effect on the indoor climate of the Rotonde through software simulations in DesignBuilder using EnergyPlus. The simulation results of multiple PCM types and thicknesses showed that the current indoor temperature of the Rotonde is too inconsistent during summer months for a PCM to reach the intended cooling effect. A simulation of a combination of improvements to the building envelope and the most effective PCM showed a cooling effect of two ° C on warm summer days and a decrease of 50% in heating demand during the winter. This research amplifies the importance of a well-insulated building envelope for a qualitative indoor climate and climate systems should be an additional step. The outcome of this research has the potential to be relevant for other monumental museums or other spaces that seek sustainable solutions to stabilize the indoor climate.