In order to achieve the national goal of greenhouse gas emission reduction in building sector, it is essential that insulation performance of existing buildings should be improved. Because old buildings built before 2001 accounts for 73.2% of total buildings in Korea, when the building insulation regulation was highly tightened. The aim of this study was to evaluate the actual insulation performance of opaque outer walls of old commercial buildings by field measurements. A variety of standards and relevant research which deal with the field measurement of thermal transmittance were surveyed. Outer wall U-values of old commercial buildings were measured and compared with the required value prescribed in the insulation regulation enforced when evaluated buildings were designed. As results, actual insulation performance was found to be approximately two-thirds of required level prescribed in the insulation regulation.