This paper reviews sustainable retrofitting strategies for low-rise apartment building envelopes in the composite climate of New Delhi, emphasising the need to reduce cooling energy use, improve indoor thermal comfort, and contribute to environmental sustainability through reduced carbon emissions. A systematic review of literature analysed several related studies, government reports, and policy documents from both national and international sources. It highlights envelope-related retrofit strategies, such as wall and roof insulation, reflective coatings, advanced glazing, and passive cooling methods. The review also covers simulation tools such as Design Builder and TRNSYS, as well as regulatory standards like the Residential Envelope Transmittance Value (RETV), used to assess retrofit effectiveness. The review highlights consistent evidence that envelope retrofitting can significantly reduce cooling energy demand while enhancing thermal comfort. It also identifies significant research gaps, including the limited availability of region-specific models for composite climates, underutilisation of RETV in design validation, insufficient integration of occupant behaviour, and inadequate attention to cost-effectiveness frameworks. The synthesis of existing literature provides valuable guidance for researchers, practitioners, and policymakers seeking to adopt envelope-focused retrofitting approaches. It highlights the importance of climate-responsive guidelines, simulation validation, and socioeconomic factors in future retrofitting efforts for India’s residential sector. By consolidating dispersed findings into a structured framework, this review advances the body of knowledge on retrofitting strategies in composite climates. It offers a critical evaluation of current research, identifies unaddressed gaps, and suggests future directions for developing region-specific, cost-effective, and occupant-centred retrofitting models. Overall, it will demonstrate the connection between building performance improvements and environmental sustainability objectives, as well as climate-responsive design.