Optimum Insulation Thickness for Building Exterior Walls in 32 Regions of China to Save Energy and Reduce CO2 Emissions

China accounts for approximately 30% of the CO2 emissions—one of the largest proportions from a single nation. Therefore, reducing the CO2 emissions in China will play a decisive role in global CO2 emissions reduction. This paper is a proposal of optimum insulation thickness (OIT) for the exterior walls of buildings in 32 regions of China to save energy and reduce CO2 emissions. The determination of the OIT for exterior walls its impact on the annual total energy cost and annual CO2 emissions per unit area of building exterior walls were calculated for 32 regions of China using the life-cycle cost analysis (LCCA) and degree-days (DD) method. The results indicate that the annual total energy cost decreases up to the OIT level, and increases beyond the OIT level. The annual CO2 emissions can be reduced by increasing the insulation thickness. It is predicted that the average total energy cost of 32 regions can potentially be decreased by about $5/m2-year (40%) and the average CO2 emissions of the 32 regions can potentially be reduced by about 27 kg/m2-year (63%) when the OIT is applied. It also shows it is more effective to apply the OIT for the exterior walls of buildings to save annual total energy cost and reduce annual CO2 emissions in the Severe Cold and Hot Summer & Cold Winter climatic zones rather than the other climatic zones of China.