Analytical procedures for estimating airflow rates in ventilated, screened wall systems (VSWS)

The wetting, storage and drying of moisture is a serious concern in the overall performance of exterior wall systems. Prediction of moisture transport and moisture removal within wall enclosures plays a key role in the design of exterior wall systems. The convective drying potential in ventilated and screened wall systems (VSWS) is directly proportional to the available ventilation flow rate of the air through the vents in the walls. The main goal of this study was to determine the most appropriate analytical equations for predicting ventilation flow rates within VSWS and to provide a solid foundation for estimating convective drying potential. The equations currently available in the ASHRAE literature are primarily intended for mechanical system applications and are not well suited for estimates of convective drying potential in wall systems, mainly due to the extremely low velocities and the unique geometry of VSWS when compared to mechanical systems. Other literature sources were also reviewed for the evaluation of pressure losses in systems with low airflow rates. The pressure losses due to friction at the walls of the ventilated chamber were beyond the scope of this paper. Under most circumstances, the major contribution to pressure losses in VSWS occurs at the inlet and outlet vent openings. A list of the empirical/analytical equations recommended for use in evaluating natural ventilation flow characteristics within VSWS is also provided. The recommended equations provide a solid theoretical background for validation against the laboratory and field results obtained in the ASHRAE Research Project RP-1091.