The FEM-analysis of factors influencing the heat transfer properties of external walls made of autoclaved aerated concrete blocks is presented. Using the ELCUT program, the external wall temperature fields made of autoclaved aerated concrete blocks were calculated for different values of thermal conductivity for laying cement mortar and laying with adhesive. The temperature field was calculated for a junction of a top floor slab with a two-layer external wall having a facing brick layer and no air cavity. The possibility of condensation of moisture on the surface of the ceiling is established. The values of heating performance uniformity of heat productivity, reduced thermal resistance and heat fluxes for external walls are determined. The calculated values of heating performance uniformity of heat output are obtained depending on the coefficient of thermal conductivity of autoclaved aerated concrete blocks. Further development of the research will be the use of data in calculating the payback period of energy efficiency measures for buildings.