Building energy plays a significant role in total energy consumption in China. It is widely recognized that the insulation performance of the external envelops is a critical factor for energy consumption of building air conditioning system. In this study, the effects of building external wall's insulation thickness and position on the heating and cooling loads of a commercial building studied for five cities from different climatic zones of China, namely, Harbin, Xi'an, Shanghai, Kunming and Guangzhou, are investigated numerically. The wall's optimum insulation thicknesses of the building simulated in these cities are determined by the life cycle cost analysis (LCCA) method. Meanwhile, the different positions of insulation layer embedded in the wall are investigated in terms of the time lag and decrement factor. The results show that the increase of insulation thickness has a significant effect on the building heating load, inversely it exhibits a relatively small effect on the building cooling load. The analysis indicates that building energy savings vary from different climatic zones. For a given wall insulation and the same building conditions, the largest building energy savings are achieved in Harbin, and energy savings of other cities follow the order of Xi'an, Shanghai, Kunming and Guangzhou. The variation of building energy savings in Guangzhou is insignificant along with the increase of the insulation thickness. Using expanded polysttyrene as insulation layer material, the optimum insulation layers of the building in Harbin, Xi'an, Shanghai, Kunming and Guangzhou are founded to be 80 mm, 60 mm, 40 mm, 40 mm, and 20 mm, respectively. (C) 2015 Elsevier Ltd. All rights reserved.