Even if flat glass is 100% recyclable, only 1% is currently handled in circular or closed-loop supply chains (CLSCs) in Sweden. This has an unnecessary environmental impact, indicating not only the potential for more energy-efficient solutions but also the challenges that need to be understood. The purpose of this article is to increase the knowledge of challenges in applying different types of more or less energy-efficient CLSCs for flat glass. Through a literature review, an overview of CLSC types, together with challenges in different areas, is provided. The CLSC types and challenges are corroborated in a flat glass context, including focus groups and expert interviews. Four CLSC types-two CLSCs based on remanufacturing, one on reconditioning, and one on reuse-are identified. A framework provides implications for both literature and practice. It contains 19 challenges-such as lack of large-scale actors, lack of material knowledge, lack of knowledge of customers' behavior, lack of promotion of flat glass CLSCs at many levels in society-in terms of both legislation and cost-and lack of business models-structured in four areas-material characteristics and quality, inefficient logistics systems, demand and supply, and means of control and costs. The least-applied CLSC includes almost every challenge and has a large upscaling potential, indicating the necessity of mitigating challenges. The framework identifies challenges not included in earlier flat glass literature. CLSC types are related to different challenge set-ups and different energy efficiency potentials, leading to expanded CLSC knowledge. One additional implication is that practitioners can identify potential CLSC types and understand their challenges from the perspective of several stakeholders. The participatory research methodology fills a methodological research gap within CLSC literature and provides important insights.