Ventilated rainscreen cladding: A study of the ventilation drying process

The air exchange in rainscreen walls is expected to provide ventilation drying if excess moisture is absorbed in the wall construction. In an earlier paper, we presented a cavity airflow model and measurement based estimations of air change rates (ACH) in south-facing experimental walls. Here, focus is on the ventilation drying process and its practical implications. ACH in the experimental walls were calculated and converted into drying rates at different stages in the drying process. Furthermore, changes in the drying rates due to changes in the cavity design and in the outdoor climate were investigated. The significance of the drying rates was demonstrated in a case study.Findings showed that the cavity design is of major importance for the drying rate if the material adjacent to the cavity is wet over its entire extension. For such extreme cases, a light façade colour, vented horizontal battens and, in particular, a small cavity depth, are adverse factors for the drying rate. As the drying process proceeds and the moisture transport from the material to the cavity air becomes dependent on vapour diffusion, the drying rates for different cavity designs tend to be evened out. If the resistance to vapour flow in the material reaches high levels, a favourable outdoor climate is significantly more important than the cavity design to promote drying. Also for the case where the material adjacent to the cavity initially is only locally wet, findings showed that the cavity design is of minor importance for the drying time.

► We studied relations between cavity design, air change rate and ventilation drying.
► Calculated air change rates compared well with estimations based on experiments.
► Ventilation drying rates were calculated at different stages in the drying process.
► The significane of the ventilation drying rates was demonstrated in a case study.
► A small cavity depth (<10 mm) prolongs the ventilation drying process significantly.