Heat transfer through the exterior building envelope in real operating conditions is always unsteady. However, in practice, in most cases, steady-state heat transfer is discussed, characterized by the time-constant magnitude of the heat flow rate and temperatures. The steady-state heat transfer equations are greatly simplified. This makes it practical for developing engineering calculation methods. Modes of non-stationary heat transfer also find practical application. However, these methods have a number of problems. The authors proposed a method for solving the modes of unsteady heat transfer, based on probabilistic methods of the general theory of transference. The paper considers the heat flow rate through a flat building envelope consisting of several successive layers. We showed how the order of the layers in the composition of the multi-layer building envelope affects its thermal stability. We obtained an equation for determining the difference between the average times of passage of heat flow through the building envelope at various layers of disposition.