Abstract

Rain screen wall systems are effective for control of rain water. The ventilation in the air space of such walls is also expected to play a role in the drying of cladding and control of vapor flows through the wall. For example, wet cladding exposed to solar radiation may subject the wall to inward vapor flow, unless an air cavity short-circuits that vapor flow. Although the role of the air cavity has been previously studied, it is relevant to evaluate more quantitatively the movement of air within the air cavity between the cladding and the backwall. The effect of wind pressure notwithstanding, convective movement of air in the cavity is the result of buoyancy. Air movement can result from the increased temperature of the cladding due to solar radiation. The first objective of this thesis was the development of a numerical model of air movement driven by the variation of wall temperature with height, taking into account heat transfer by conduction, convection and radiation and mass transfer by diffusion and convection. The model was validated by means of comparison with analytical results from the literature, as well as with two sets of experimental results. The experimental procedures involved the evaluation of the magnitude and direction of the air movement within a large-scale cavity with a uni-directional anemometer and with particle image velocimetry. Once the boundary conditions were known, they were used to determine experimentally the mass transfer coefficients at the surface of wetted brick in the cavity with a horizontal wind tunnel setup. The validated model was used to evaluate parametrically the air movement's effects on heat and moisture flow within the assembly, assuming the brick had wetted by rain water before exposure to the solar radiation. Conclusions were drawn as to the effect of the air movement on the evacuation of heat and moisture from the assembly, based on variation of six parameters. It was found that the air movement in the cavity plays a significant role in the drying of the cladding by saturation of rain water. As well, under average summer conditions for Montreal, the air velocity in the cavity was found to have an approximate peak of 0.20 m/s, neglecting the effects of wind pressure. The air flow is turbulent at the weephole entrance of the cavity, before becoming laminar within approximately 10 centimeters of flow development. Finally, convective mass and heat transfer surface coefficients increase with air velocity, except for air velocities lower than 0.10 m/s