The temperature distribution of an outer wall of a room subjected to solar radiation goes through a transient state until it reaches a thermal equilibrium state. After an interruption of solar radiation, the temperature profile on the wall cross-section, starting from the latter thermal equilibrium state, returns through a transient state to the initial thermal equilibrium state. The transient state between the above two equilibrium states is called the thermal restoration transient state of the wall. The duration of the thermal restoration transient state depends on the structural characteristics of the wall and on the thermal parameters of the room-environment system. The influence of the thermal restoration transient state on the evaluation of the ability of an opaque wall to store and exploit solar radiation and on the computation of the resulting thermal gain of the room during the thermal restoration of the wall is important, because it acts as a flywheel. In the present work analytical expressions yielding the temperature profiles and the duration of the transient state as a function of thermal and structural characteristics have been developed. An analytical model for the thermal gain of the room during the thermal restoration of the wall also was developed. An experimental verification of the duration of the transient state of the thermal restoration was made.