This paper presents a modeling methodology for an engineering calculation of the thermal stability of external walls with a shielded outer surface forming a ventilated façade system. The objective of the study is to develop a practical design tool for assessing the amplitude attenuation and phase shift of the internal surface temperature under daily climatic fluctuations. The method is based on the solution of a one-dimensional transient heat conduction problem for a multilayer structure subjected to periodic climatic effects. The external boundary condition is defined through an equivalent heat transfer formulation that accounts for shortwave solar radiation, longwave radiative exchange between the screen and the ambient environment, convective heat transfer, and possible ventilation of the air cavity. An engineering calculation algorithm is proposed that incorporates the effect of equivalent solar loading and harmonic variations of outdoor air temperature with high amplitudes of environmental and near-wall air layer fluctuations. A numerical procedure is provided for the “screen – air gap” subsystem, followed by the evaluation of internal surface temperature attenuation and transient heat transfer characteristics. Validation against numerical simulations and experimental data demonstrates a deviation not exceeding 5-10%. The results indicate a significant influence of screen reflectivity, air gap ventilation intensity, and wall heat capacity on improving thermal stability and reducing heat gains during the hot season. The proposed enhanced assessment algorithm can be widely applied in design practice, including the selection of thermal insulation thickness for building envelopes in southern regions, the determination of design loads for ventilation and air-conditioning systems, and the evaluation of indoor thermal conditions under intermittent heating and ventilation regimes. The practical significance lies in the ability to optimize façade system parameters with shielded external surfaces to prevent overheating and improve building energy efficiency. The study is conducted within the framework of ensuring thermal safety of buildings in warm climate conditions.