Apartment balcony has been indiscreetly remodeled since the government permitted remodeling on January 2006. But remodeled balcony has a few problems such as increase of heating energy, surface condensation and cold draft. The reason of thermal problem is mainly caused by the window system in a extended balcony. The purpose of this study is to analyze heating and cooling energy and propose the efficient window types for the extended balcony area of a apartment building. 4 types of window system which have fairly high U value in Korea are investigated as follows ; double clear glass, double low-e glass, triple clear glass and triple low-e glass. Comparing double clear 91ass with double low-e glass, triple clear glass and triple low-e glass, simulation results show that 10%, 7% and 15% saving of total primary energy can be expected.