Solar thermal collectors play an important role in improving the performance of low-temperature thermal systems by reducing heat losses and material-related environmental impacts. This study investigates the thermal and sustainability performance of glass tube solar collectors (GTCs) by jointly analysing heat loss mechanisms, structural complexity, and life-cycle indicators relevant to sustainable engineering design. Four GTC configurations are examined: single-glazed collectors with air and vacuum insulation (SG+Air, SG+Vacuum) and double-glazed collectors with air and vacuum insulation (DG+Air, DG+Vacuum). An analytical model based on iterative thermal resistance calculations is developed to quantify coupled radiative, conductive, and convective heat losses through multi-layer cylindrical glass structures under representative operating conditions (ambient temperature 10–30°C, absorber temperature 40–90 °C). The thermal analysis is complemented by a multi-criteria decision-making framework using SAW and TOPSIS methods, incorporating geometric, economic, and environmental indicators, together with a simplified life cycle assessment. The results show that the single-glazed vacuum-insulated configuration reduces heat losses by up to 57.4% compared to air-filled designs, leading to improved thermal performance in low-temperature applications. Life-cycle results indicate that this configuration also exhibits the lowest embodied energy (160.3 kWh) and global warming potential (22.1 kg CO₂-eq), demonstrating a favourable balance between operational performance and material-related environmental impacts. The proposed framework provides a practical basis for the performance-oriented optimisation and sustainable design of glass tube solar collectors, supporting the development of green thermal energy technologies.