An effective analytical method for analyzing the transient thermal performance of building envelopes is very important to optimize their thermal performance design. However, the influence of thermal mass and insulation material distribution cannot be analyzed well by using the current steady state thermal resistance (i.e. R value) and thermal inertia index (i.e. D value). Moreover, the substitution effect between the thermal resistance R and the thermal capacitance C has not been theoretically studied before. A novel analytical approach based upon thermal impedance of evaluating the transient thermal performance of building envelopes is developed in this paper. By using that, the optimal material distribution (corresponding to the minimal space heating or cooling load due to building envelope heat loss or gain) can be analytically determined. In addition, the substitution relationship between R and C can be theoretically derived. The results show that putting concrete material in the middle and evenly distributing the insulating materials on the inside and outside surface can maximize the module of thermal impedance for given conditions. The proposed approach provides an effective way to theoretically optimize the transient thermal performance of building external wall. (C) 2014 Elsevier B.V. All rights reserved.