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.

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.