Disputes and repairs due to window condensation problems frequently occur after the completion of apartments. However, most repairs are performed individually and confidentially by construction participants, who are forced to minimize economic losses. Thus, actual cases of inspection and repair for condensation defects have rarely been addressed in public even though condensation on windows remains the primary cause of defect disputes. This study aims to contribute to the resolution of similar disputes by presenting an actual case of a new apartment complex in which defect disputes arose from window condensation; the condensation resistance performance was inspected, and repair measures were proposed. On-site measurements and mock-up tests were performed. The results demonstrated that the U-factor satisfied its required specifications but that the temperature difference ratio did not satisfy the requirements. Repair measures were proposed and evaluated; the combination of adding sealant, double glazing near the existing glazing, and an insulation cover on the frame was found to be effective. These repair measures were applied to the apartment complex and the number of condensation complaints decreased significantly.