In the current context of a necessary reduction in environmental impact, the reuse of heritage buildings is key. Although architectural rehabilitation already significantly reduces this impact, thermal comfort facilities present several challenges, both due to the huge visual impact they entail, compromising buildings’ historical values, as well as the environmental impact due to machines and ducts and the operating energy consumption. This paper demonstrates, through the case study of the Rocafort de Queralt Wine Cellar, that it is possible to reduce to nearly zero the thermal comfort facilities of converted heritage buildings for public cultural use. This historic building, considered one of the “Wine Cathedrals” in Spain, was built in 1918 and is characterized by its architectural style typical of Catalan modernism. The method applied was in situ monitoring, combined with dynamic simulation with DesignBuilder v7 software and NECADA software v2024, considering four different scenarios of the building: (1) the current state; (2) after passive improvements; (3) after architectural rehabilitation; and (4) subject to the climatic conditions expected for 2050 according to IPCC AR4 A2. The conclusions are surprising. In Scenario 2, 87% thermal comfort is reached with zero facilities, and 100% thermal comfort is reached when the location of cultural activities within the building is changed according to geographical orientation and the season of the year.