This study examines standard series of panel residential buildings, evaluates their energy-efficiency indicators, and develops recommendations for improving the thermal performance of external wall assemblies. All calculations were carried out for nine standard residential building series located in St. Petersburg. The thermal engineering characteristics of the building envelope, such as thermal resistance, required insulation thickness, and heat losses through the external envelope were determined. Thermal engineering calculations, along with the assessment of the payback period for each proposed insulation option, were performed using Microsoft Excel. Taking into account the growth rates of heating tariffs and the implementation of energy-efficiency measures, the least favorable option in terms of payback period for all building series is the use of a wet facade with EVERGEL insulation. The shortest payback periods for most building series are achieved when applying a ventilated facade system, except for series 1-468, for which the optimal solution is a wet facade with TeploKnauf Stena Extra insulation; and for series 1-528KP, where the shortest payback period is obtained using a ventilated facade without insulation. A greater difference in heat losses through the external envelope before and after insulation (ΔQ, rub/m2) corresponds to a more energy-efficient wall assembly.