The energy consumption of buildings is huge and the thermal properties of external walls have an important influence on building performance. Traditional researches on the subject, where numerical methods are always used, focus on linear systems and neglect the variation of the wall’s thermal properties with temperature. The concept of entransy is introduced in this paper. It is found that the ideal thermal properties of wall corresponds to the equivalent heat transfer resistance extremum in the heat transfer domain from outdoor to indoor side. An analytical method based on the entransy theory is put forward. By using this method, we can obtain the theoretical solutions of the ideal variation function of thermal properties with the wall temperature. The results show that the ideal heat capacity ρcp(T) of wall is a δ function in summer, the ideal thermal conductivity k(T) of wall approaches a staircase function in summer. Comparison with the numerical method shows analytical method has advantages in calculation accuracy and speed.