Most of the existing buildings in Korea, constructed before the insulation standards for parts were strengthened, are requested by an act to improve the building energy performance grade. In this study, five old public buildings were examined to evaluate their energy efficiency rating using an in-situ analysis of each old public building. Energy efficiency measures were derived to achieve the zero-energy level of the buildings’ energy performance based on the energy consumption analysis results using ECO2 and the actual building status survey. It is expected that all buildings to be analyzed will be able to achieve ZEB 5th grade when applying the appropriate remodeling measures considering the characteristics of each building. To effectively reduce greenhouse gas emissions, energy-intensive buildings need to be selected as the target of the green remodeling project as a top priority. In the case of buildings with a 1++ grade, current energy efficiency is excellent to date. Therefore, it is necessary to improve the overall building performance by improving the lack of partial performance.