This work proposes an innovative integrated retrofitting system aiming to improve both the seismic and energy performance of existing reinforced concrete and masonry buildings. The system is based on engineered insulating concrete form panels, installed on the outside of existing buildings as a shell exoskeleton. A key major advantage of the proposed system is that it addresses the contemporary improvement of seismic and energy performances of existing buildings in a single installation stage, operating exclusively from outside of the building. The insulating formworks are ad hoc prefabricated in a factory on the base of the specific geometry of the existing buildings so as to greatly maximize the ratio between overall retrofitting benefits and costs and at the same time to simplify the installation procedures. The objectives of the presented research are, on one hand, to highlight the major structural issues that the system aims to address, and on the other hand to illustrate the main characteristics and combined benefits of the proposed retrofitting system. From a structural point of view, the proposed system is conceived to behave as a non-dissipative structure with regard to seismic actions, and the lateral strength and stiffness of the structural elements are designed accordingly. An analytical design approach is proposed and validated using the available data from an experimental test performed on a full-scale simple building. Moreover, numerical modeling strategies for the proposed system are illustrated for two complex case study buildings. The results of the analyses show a considerable increase in lateral stiffness of the retrofitted buildings that, considering the non-dissipative behavior of the elements, leads to a relevant reduction of seismic deformation demand on existing structural elements.