A well-designed ventilated wall that incorporates air-space behind cladding can reduce energy use for conditioning buildings by adding a thermally resistive layer. The scarcity of thermal performance data and standardized test methods is a particular barrier for practitioners to implement ventilated air gaps behind the facades. The current standardized test method to evaluate the thermal performance of a wall structure, such as ASTM 1363-19, does not consider the effect of the ventilated air-space behind external claddings. Therefore, testing and design recommendations are required to account for the impact of ventilation airflow on the thermal resistance of vertical air-spaces behind common cladding materials. In this study, the theoretical uncertainty analysis of the effective thermal resistance of the ventilated cavity is performed for a steady-state condition. The results revealed that temperature sensors with an absolute uncertainty of ±0.18°F (±0.1 °C) and heat flux sensors with a relative uncertainty of 3% or lower are required to capture a wide range of the effective thermal resistance of the ventilated air gap. Thereafter, based on the uncertainty of temperature and heat flux measurements, the modifications of the ASTM C1363-19 test method to account for the airflow effects on the thermal performance of the wall assembly are proposed. A detailed description of the experimental setup is provided, and protocols for collecting, analyzing, and evaluating data are suggested. An example demonstrating how the proposed method can be practically used to measure the thermal resistance of a ventilated air-space is also presented.