The amount of heat flux passing through the building envelope directly affects the energy use for the thermal conditioning of the building. The ventilated cavity behind the traditional external claddings (e.g., vinyl siding, brick veneer) adjacent to the wall core can contribute to the magnitude of the heat flux entering or leaving the indoor space. Thus, the effect of the ventilated air-spaces on the variation of the heat flux passing the wall core is investigated in this study. A numerical code is developed for the steady-state condition and verified with available in the literature hot box tests. Several simulations are performed, and the effects of the air change rate in the air gap (0 to 1000 1/h), seasonal variation (summer and winter conditions), and external cladding type (brick and vinyl siding) on the heat flux passing through the wall core are discussed. The results show that the ventilated air-spaces have an impact on the thermal performance of the entire assembly.