Cities are fast growing and are becoming more and more dependent on external services and supplies to meet their demands. In this context, the building industry can play a major role to reach European and regional targets of low-energy construction and circular design, particularly if considering the need of improving the energy standards and performance of the existing stock. On these premises, this paper presents the findings from recent research and discusses upcoming challenges, in particular: what are the consequences of retrofit operations on material flows, and their impact on circularity and low carbon objectives? In other words, which energy retrofit strategies are being implemented in buildings that could contribute to the production of building’ waste or secondary resources today, but also in the future? The purpose of this paper is to analyse the impact of different retrofitting scenarios on the energy performance of material flows. These scenarios can directly influence the nature and quantity of the materials used (inflows) and discarded (outflows) by upgrading or renewing the existing building stock. They can also lead to different environmental impacts and vary the embodied potential (through reuse or recycling) of resources. The analysis focuses on selected case studies representative of the housing stock in Brussels (Belgium). The overall objective is to inform, sensitize, and lead various stakeholders to responsible and conscious choices when retrofitting a building by adding concerns of resources efficiency while focusing on reducing energy demands.