This paper shows the influence of mortar joints on the thermal resistance of external walls made of autoclaved aerated concrete blocks. Due to the fact that the thermal conductivity of cement mortars and adhesives is significantly higher than the thermal conductivity of autoclaved aerated concrete products, their effect must be considered when designing the walling. The article contains a table of thermotechnical uniformity coefficients of aerated concrete walls depending on the thickness and thermal conductivity of mortar joints. The work describes the simulation of heat transfer in stationary and non-stationary conditions. In the case of unsteady heat transfer, the influence of masonry joints on the thermal resistance of the enclosing structure was considered. Thermal resistance was characterized by the range of temperatures and the estimated damping of range of temperatures. There was calculated a correction value for the effect of mortar joints on the complex damping coefficient of estimated range of temperatures.