Estimation of cavity pressures in air-permeable, multi-layer systems using a lumped-leakage approach

For conventional buildings, the proper estimation of wind-induced pressures on the external façade, the roof surface, or the net pressures across elements like a canopy or parapet, which are exposed to wind on both sides, can be easily done using conventional wind tunnel tests. But in the case of air-permeable multi-layer systems, which have gaps or porosity in the external layer along with a cavity between the external and inner layers, the estimation of wind loading across the external layer or in the inner cavity by wind tunnel tests can be quite difficult due to practical difficulties in exactly simulating the dimensions of the gaps in the physical model with small model scales or other practical issues related to the tubing of the pressure sensors. Pressure equalization plays a major role on the wind loading on individual members of multi-layer systems and in this study, an analytical model to estimate the time- varying cavity pressure distributions in a double-layer system with an air-permeable outer layer was developed, given the external pressure on the outer surface. The pressure drop associated with the flow through the gaps in the external layer was modeled using orifice flow equation and mass conservation equation (Oh. J.H. & Kopp, G.A., 2014). The model accounts the geometric parameters like the cavity depth (H) which is the distance between the outer layer and the inner non-porous layer along with loss coefficient for the orifice flow through the gaps in the external layer. Moreover, the pressure drop due to flow through the gaps (G) in the external layer are accounted based on a lumped-leakage approach. The results from the analytical model are compared with wind-induced loads obtained from the wind tunnel test of roof-mounted photovoltaic solar array system with high G/H ratio obtained from Stenabaugh (2015)