Recently, urgent needs have arisen for improving the fuel economy of passenger cars. To improve the fuel comsumption, it is necessary to develop a technology that can improve fuel efficiency and weight-saving. This paper describes the development of a soundproof package to balance weight-saving with performance of acoustic isolation used to reduce engine noise. First, we developed a hybrid statistical energy analysis (HSEA) model to evaluate the performance. Second, by using the HSEA model, we (1) analyzed the power flow and dominant path from noise source to interior cavity, (2) extracted efficient sections such as dash, dash penetration parts, and floor so as to improve the performance. Using the above process, we developed a soundproof package that improves the performance without increasing the weight. As a result, we balanced weight-saving with performance of acoustic isolation using the HSEA model.