The determination of building insulation levels is an indispensable part of low-energy residential building designing. It is necessary to study the impacts of varying insulation levels of outer walls and roof on room heating loads and cooling loads per square meters. Although diversity of researches on this subject have been carried out over the past years, simulation models have not been employed to evaluate the energy saving effect of different insulation levels all-year, and the results were not quantitatively analyzed. In this paper, some dynamical evaluation results for a prototypical residential building in Beijing are presented by means of the DeST building simulation program. Finally, some practical solutions are suggested.