Over several decades, insulation materials have served the crucial role of meeting thermal performance standards for buildings. However, a contemporary challenge arises as an increasing number of these aged insulation materials are released now during demolition or renovation. Unfortunately, these materials always find their way to landfills or burning furnaces, while they are not yet at the end of their lifespan. To tackle this challenge, there is a growing need to explore the potential of reusing insulation materials to close material loops and reduce the ecological footprint of the current building industry. To initiate the reuse of insulation materials, there is currently a lack of knowledge regarding performance and degradation assessment after natural ageing and dismantling, which is necessary to identify the potential reuse applications. Therefore, this paper focuses on the material characterization of naturally aged polyurethane (PUR) and polyisocyanurate (PIR) foam insulation dismantled from various applications (e.g., roofs, walls, etc.). While standardized test procedures exist for new insulation products, assessing naturally aged materials requires a more nuanced approach. By prioritizing the most critical performance criteria for reuse, only relevant tests are conducted on the collected insulation samples (water absorption, thermal conductivity, compressive stress and dimensional stability). The initial results indicate that reusing PUR and PIR insulation is certainly feasible; however, certain performance aspects require a safety margin to be included. These outcomes not only provide insights into potential degradation phenomena, aiding manufacturers in improving the durability and service life prediction of their products, but also lay the groundwork for estimating their reusability potential. Ultimately, the findings contribute to the development of guidelines and tools for stakeholders to responsibly manage reuse, thereby promoting circular and sustainable construction practices.