In current climatic and social context, energy retrofit of existing buildings plays a really important role thanks to the considerable economic and CO2 savings connected to the lower building energy demand. This study fits perfectly into this context and aims to evaluate the effects of an energy and structural retrofit intervention on an existing building representative of the pre-1980s Italian period by means of a steel reticular exoskeleton. This exoskeleton was designed in order to improve the dynamic response of the existing structure and to realize a new high-performance double-skin unitized façade. The new façade, designed taking into account fundamental geometrical criteria, creates a floor-by-floor air volume, used as a greenhouse in winter and as a ventilated cavity during summer. Furthermore, the choice of low-emissivity performance glass for the wide glazed surfaces allows to reduce the heat losses during winter, reducing the overall energy demand. During summer, on the other hand, the activation of solar-controlled automated venetian blinds, combined with the control of natural ventilation, allows a significant reduction of the cooling demand. The transition from winter to summer mode is guaranteed thanks to a dedicated schedule that allows the control of solar radiation and natural ventilation from May to September. From a performance point of view, thanks to the control and optimization of the natural ventilation and incident solar radiation in the double skin cavity, the overall energy consumption of the existing building can be reduced by more than half. In conclusion, this study highlights how the proper design of an exoskeleton allows to both improve structural and energy behaviour; in particular, using the exoskeleton to create a glazed double skin façade, significant savings can be achieved, making existing buildings less energy-intensive.