This study analyzed the current airtightness performance of conventional buildings in Korea and proposed improvement strategies focusing on major leakage points, particularly external service penetrations. Airtightness was measured in approximately 280 buildings of various types and completion years using the Blower Door Test method in accordance with ISO 9972. The buildings were classified as low-energy or conventional depending on whether airtight design and construction elements—such as sealing tapes, membranes, and gaskets—were applied. The analysis revealed that the airtightness performance of conventional buildings varied widely, ranging from 0.56 h-1 to 24.81 h-1. Although more recently constructed buildings tended to exhibit slightly improved airtightness, a significant performance gap remained when compared with low-energy buildings. Field inspections identified the primary leakage sources as external penetrations—including plumbing and electrical service openings in ceilings and walls—where inadequate sealing and incomplete finishing allowed outdoor air to infiltrate. To address these issues, detailed airtight construction methods were proposed, emphasizing the use of soft foam filling, high-performance sealing tapes, and specialized gaskets to ensure continuous airtight layers around service penetrations. These measures are expected to significantly reduce infiltration through building envelopes and improve the overall energy efficiency of existing conventional buildings. This study provides a quantitative overview of airtightness performance in Korean buildings and highlights practical construction details for improving weak points in existing structures, thereby contributing to the advancement of energy-efficient and carbonneutral building practices.