Abstract Many Korean apartment units use balconies as semi-indoor spaces by installing external windows. These balconies form environmental buffer spaces to improve comfort and reduce cooling and heating costs. However, the balcony spaces of some units have been removed and have been placed in living rooms. The removal of these environmental buffer spaces has caused problems. Significant heat loss through windows is one of the most serious problems; window surface condensation caused by low surface temperatures in the winter makes residents uncomfortable and damages construction materials. Therefore, it is evident that window design requires improvements. This study introduces a double window system with improved thermal performance and ventilation slits to solve the window surface condensation problem in apartment units without balconies. To verify the thermal performance of the proposed system, temperature factors were analyzed by field measurements. The condensation frequencies were also evaluated by temperature factors at an indoor temperature of 25 °C and 55% humidity. Ultimately, the double window system was able to reduce condensation frequency greatly. An examination of the air volume by ventilation slit operational mode found that the case with an open outer window bottom vent and an inner window top ventilation slit, discharged the indoor vapor effectively and increased the dew point of the window's surface.