The task was to to carry out thermal simulations of constructionaldetails of the external space-delimiting structures of the Buda Castle Palace's Saint Stephen's Hall and it's wing before reconstruction. Due to the prominent historical character of the building, the wing was built with non-insulated walls, with a solid brick wall between the reinforced concrete frame structure and external stone cladding. Solid brick was suitable for the original building use, however, the climatic requirements imposed by the new function, a museum, required strict temperature and humidification controls. In combination with uninsulated structures, this could result in mould growth or even condensation. It was necessary to examine which structural details should be provided with additional thermal insulation, depending on the given design air condition, and how, of course, without changing the external and internal appearance. During the project, we performed and analyzed 2D and 3D stationary thermal engineering simulations, and based on these results, we made recommendations for constructional changes.