If buildings can change "clothes" according to seasons like human being, the energy consumption for their space heating or cooling will be saved greatly. For this purpose, the thermophysical properties of building external wall should be self-adjusted according to outdoor temperature. In this study, a method of determining the ideal thermal conductivity k(t) of external wall with constant volumetric specific heat rho c(p) is developed based on the concept of ideal passive energy-efficient buildings. As illustration, a passive room in Beijing is analyzed. The result shows that the ideal k(t) of external wall is approximately a square wave function whose type is hardly influenced by rho c(p) and the thickness of external wall, L. After optimization, the integrated uncomfortable degree has reduced by 64.3% compared with the traditional external wall. The approach can also be used in the multilayer wall. (C) 2013 Elsevier Ltd. All rights reserved.