Ventilated rainscreen cladding: Measurements of cavity air velocities, estimation of air change rates and evaluation of driving forces

To analyse the moisture performance of wall systems with a ventilated rainscreen cladding, the air change rate per hour (ACH) is required. However, the average ACH and its variation depend on many factors. This study focuses on performing field measurements of air velocities and temperatures in south oriented wall cavities characterised by either vertical wooden battens or horizontal vented metal battens. A physical cavity airflow model together with laboratory test of loss factors were used to analyse the data and interpret the results. With vertical battens, findings estimated the average ACH during a measurement period to be 230–310 ACH. In the cavities with horizontal battens, the ACH was 60–70% lower. The daily variations were considerable and hours with solar radiation and clear skies resulted in ACH that exceeded the average values 2–3 times. In contrast to airflow induced by thermal buoyancy, wind-induced airflow was irregular with frequent changes in both velocity and direction. This pattern was observed independent of the angle between the wind and the cladding. The frequent changes in flow direction significantly reduced the efficiency of wind-driven airflow to create air exchange. The wind-induced airflow in wall cavities with a pronounced non-linear relationship between the driving force and the air velocity is suppressed in the presence of buoyancy. For rainscreen claddings exposed to many hours of solar radiation, this effect increases the possibility of accurate estimations of ACH.