The presence of an air-space within a building envelope is known to have a varying contribution to the overall thermal performance of the wall assembly due to the combined effect of convection and radiation in the air cavity. In particular, the thermal resistance of a ventilated air-space can vary significantly depending on multiple environmental and thermo-physical parameters. Although the thermal resistance of enclosed air-spaces in the building structures has been thoroughly investigated in the literature, it has not been defined for a ventilated cavity. This paper aims to introduce the plausible definitions of the thermal resistance of a vertical ventilated air-space behind external cladding systems. Both theoretical and applied formulations are provided and compared. The energy balance method is used to model the steady-state heat transfer through two types of traditional external wall systems (i.e., brick and vinyl siding) in summer and winter conditions. A range of air exchange rates in the cavity is examined, and the effect of the presence of reflective insulation in the air-space on the thermal resistance of the air gap is analyzed. The results show that the presence of a ventilated cavity in the wall assembly can influence the thermal performance of the building envelope. In particular, the effective thermal resistance of a ventilated air-space behind a brick cladding wall could be between 0.17 and 1.85 times the thermal resistance of the cladding in the range of air change rate in the cavity from 0 to 100 1/h. The effective thermal resistance of the ventilated air gap behind vinyl siding could reach up to 9 times the thermal resistance of the cladding.