Recently, the coverage of urban forests has been rapidly decreasing as the cities are created and expanding. Consequently, there arise urban problems such as heat island effect, urban flooding, urban desertification and so on. In this context, green roof systems is considered to be an efficient alternative to deal with these problems. However, it is difficult to apply green roof to new buildings since the majority of the buildings in cities are already constructed and the demand for new building constructions is not high enough. Therefore, it should be considered to apply green roof system to existing buildings for resolving various problems. This study evaluates heating and cooling energy consumption based on the combination of passive design factors such as wall, roof, window insulation in addition to a green roof system applied to an existing house by using an energy simulation program. Total 8 potential improvement cases are developed. Each case is applied to the same house with different insulation standard for simulations. Through the analysis of the simulated cases with the chosen test house, it is confirmed that heating energy consumption decreases as improvement cases are applied, but cooling energy consumption is relatively not much affected by each improvement case. In addition, when each improvement case is applied to already highly insulated house, the effect of thermal energy improvement decreases while the same improvement that is applied to the case with low insulated house tends to yield higher improvement rate.