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.

The application of circular strategies in the architecture, engineering, construction, and operations (AECO) sector has been extensively researched, demonstrating the significance of technical approaches. However, research also focuses on the organizational challenges that arise within circular networks. Recent studies emphasize the importance of collective action in fostering cooperation across the value chain to achieve circular economy (CE) goals. Nevertheless, a considerable amount of research, including EU policies, tends to concentrate on “end-of-pipe” solutions, while failing to adequately address the socio-ecological challenges inherent in the transition to a CE. This study aims to explore collective activities and work in circular construction ecosystems at the macro, meso, and micro scales, identifying their interconnections. The findings of the literature review indicate that a successful transition to a CE requires a deeper commitment from stakeholders, which is influenced by the structure and relationships within the ecosystem. The increasing complexity of these ecosystems necessitates a redefinition of stakeholder roles and competencies, emphasizing a collective perception of value. Given the lack of tools and research on collaboration, we propose developing a map of circular construction ecosystems to improve the visualization and understanding of their dynamics.

Cities are fast growing and their dependence on external services and supplies increases with time. In Europe, the existing building stock is both ageing and energy-demand intensive: its energy retrofit becomes necessary, this resulting in waste generation and pressure on the availability of raw materials. New approaches such as circular economy are arising to face these challenges. However, principles of circular economy are not necessarily associated with energy concerns. To address this gap, this paper considers the current need of retrofit strategies in an urban mining perspective. A methodology based on a bottom-up approach is proposed, aiming to evaluate the impacts of energy retrofit strategies on material stocks and flows at the building scale.