Because of the existence of beams, columns, shear walls, and other structural components, the external wall of a building is often composed of two parallel structures, wherein one is a concrete structure and the other is made of bricks. Therefore, to determine the economical thermal insulation thickness of an external wall, one can not only consider a single structure. Mathematical models of economical thermal insulation thickness considering and not considering the effect of non-homogeneity of external walls were established based on the method of life-cycle cost. The economical thermal insulation thickness and economic benefits of a building wall in Beijing that is composed of two parallel structures with EPS thermal insulation were comparatively analyzed. The comparisons were carried out with two different heat sources and three different concrete structure-to-external wall ratios. The results show that non-homogeneity of external walls have a considerable influence on the economical thermal insulation thickness and economic benefit of an insulating external wall. The economical thermal insulation thickness under the consideration of non-homogeneity of external walls will become thicker. However, the increase in life-cycle cost is not significant, whereas the increase in life-cycle saving is very significant, and the payback period becomes shorter. The effectiveness of the investment and the energy efficiency revenue without consideration of the effect of non-homogeneity of external walls are underestimated. Thus, a method that considers this effect can obtain more reasonable economical insulation thicknesses and economic benefits.