Future environmental challenges impose higher demands on energy efficiency of the existing building stock, and require that historically valuable buildings undergo changes to reduce energy use. New materials such as aerogel and eco-fiber can advance the energy efficiency of the existing building stock. Simulations of 16 different exterior wall constructions have been conducted in VIP Energy and WUFI to investigate energy savings and moisture levels. The results show that aerogel Paroc–Spaceloft 10mm lowers U-values in exterior walls by 0,27 W/m2K. The 40mm eco-fiber insulation lowers the U-value by 0,2 and 0,32 W/m2K, respectively. This means that they have the potential to energy efficiently renovate historically valuable buildings. By using the materials for exterior insulation, it is possible to improve the U-value without harming the exterior wall. The use of KC plaster as an exterior plaster has also been simulated. The use of KC plaster resulted in a greatly increased moisture level in the outer walls, as well as an extension of the drying period for the wall itself. However, the simulations show that KC plaster can protect against heavy rain better than the traditional lime plaster as it has a tighter structure.