The variation of the thermo-hydrodynamic behavior of the airflow in the ventilated cavity behind external claddings in a wall structure could potentially affect the overall thermal resistance of the entire structure. Although the impact of an enclosed air-space in the wall on the total R-value of the assembly has been thoroughly investigated in the literature, it has not been addressed for a ventilated cavity. In the present study, as a first step, plausible definitions to determine the thermal resistance of the ventilated cavity behind external claddings in transient conditions are described. As the second step, the dynamic thermal resistance of the naturally ventilated air-spaces behind passive (i.e., brick and fiber cement) claddings and active (i.e., BIPV) facades are studied using two approaches. In the first approach, a 2-D numerical model validated against measurements is employed to compute the thermal resistance of the ventilated air gap under different conditions. Accordingly, the impact of the cladding type, seasonal variation, cavity thickness, and presence of the reflective insulation in the air gap on the dynamic change of the thermal resistance of the air-space is analyzed. In the second approach, in-field measurements are performed in a test facility to experimentally examine the contribution of the thermal resistance of the cavity to the total R-value of the wall structure. The results obtained from both approaches are compared with the design values calculated based on the ISO 6946:2017 and ISO 9869–1:2014 standards. The results reveal that the dynamic change of the thermal resistance of the air gap during a day could be captured using numerical simulations. It is shown that the daily averaged thermal resistance of the cavity could reach up to 47 times higher than the design R-value of the BIPV façade. The experimental measurements confirm that the thermal resistance of the ventilated air-space could converge to a steady-state value after a certain duration of time following the requirements provided in the standards, which could be practically used for the code-compliant analysis. It is also observed that the ventilated cavity could act as an insulation layer with higher thermal resistance compared to some of the solid materials used in the wall assembly.