The Korean government is implementing a 2050 carbon neutrality initiative to handle the climate crisis; furthermore, is striving to reduce building energy consumption through zero-energy building construction for new buildings and green remodeling projects for existing buildings. The latter is primarily based on the diagnosis of the thermal performance of the existing building envelope, and this diagnosis can lead to the development of rational energy-saving improvement measures. However, in practice the thermal insulation performance value of the envelope is derived based on the calculation values on the blueprint or the legally required thermal transmittance at the time of completion, which is used for building energy analysis. In this study, the thermal transmittance of the wall was measured based on on-site diagnosis using the ISO 6946:2017 “Building components and building elements - Thermal resistance and thermal transmittance - Calculation methods”. The results were compared with those obtained from the calculation method to be utilized as a basis for analyzing the rational effects of green remodeling.