Modeling convective drying of ventilated wall chambers in building enclosures

Abstract

In North America, residential enclosure walls are often built with provision for natural convection and, therefore, provide potential for ventilation drying within the wall. At present, our knowledge of the drying process in the wall systems is limited. The drying process is driven by very low airflow rates with complex flow patterns through the narrow and irregular wall cavities, resulting mostly in unstable ventilation driven by natural convection. Different venting strategies coupled with the stochastic nature of driving forces for ventilation contribute to the complexity of the drying process. Both the physical and the mathematical modeling of wall cavity convective drying are challenging. However, it is difficult to accurately predict the convective drying rates in wall cavities at any time during the year. Nevertheless, the convective drying process is probably one of the key mechanisms for mold suppression in residential walls in the US, and therefore, needs to be fully understood and quantified. An equation is developed to estimate the convective moisture transport in screened and ventilated wall systems. The intent was to develop a simple equation for practical design applications. The equation represents a solution of the two-dimensional moisture transport equation, solved for steady state conditions assuming laminar flow with a uniform velocity field in the wall cavity. The results derived from the simple equation were compared to measured data obtained in the Building Enclosure Test Laboratory (BeTL) at the Pennsylvania State University (PSU). The comparison showed that the simple equation can accurately predict the convective drying rates. The results are highly sensitive to the environmental conditions in the immediate vicinity of the wet wall surfaces. This equation could be used in engineering practice to provide an estimate of convective drying rates in the ventilated chamber of rain-screened and ventilated wall systems. Future validation with on-site experiments is necessary.