Buildings experience significant temperature fluctuations due to heat transfer through their external surfaces, with the roof typically being the largest contributor to heat intrusion. Incorporating high-quality insulation into the building envelope is essential for controlling indoor climate, cutting down on energy demands, and reducing the environmental impact of the built environment. In response to this, our study investigates how different insulation materials and their thicknesses influence a building’s energy performance. The study focuses on eight cities across India, each representing different climatic conditions, to ensure the method is broadly applicable. By analyzing a composite insulation layer created from seven distinct insulation materials, we assess its performance using ANSYS software and e-Quest building energy modeling. Annual simulation results highlight that strategic insulation optimization greatly lowers energy consumption and associated CO₂ emissions. Energy demand reductions observed across different cases range from 30% to as high as 90%. Additionally, the optimized insulation strategies can lower indoor temperatures by up to 19 °C and cut greenhouse gas emissions by 90–96%. This strategy improves building sustainabilitySustainability while also contributing to a low-carbon future by reducing the environmental effects of energy use.