Experimental and numerical assessment of permeability functions in closed cavity façades

This paper is the second in a series on the behavior of closed cavity facades (CCF) under thermo-mechanical loading excitations. CCF are a novel trend for sustainable high quality double skin facade solutions, characterized by a minimal maintenance demand. The air cavity is designed according to stringent air tightness requirements, provided with a small dry air flow that preserves the relative humidity, minimizing the risk for condensation and dust ingress. This extremely reduced permeability, with respect to typical double skin design, gives a scenario in conflict with the general prescriptions for the structural calculations of double skins under wind and climatic loading. This paper is the second of a series of three documents that aim to identify the structural shortcomings in the current codes and to propose efficient calculation methods and modifications to the current calculation strategies. This will overcome a critical design paradox in double skin and in particular CCF aim to reach the highest sustainability performance, by means of outstanding thermal and acoustic insulation efficiency and providing the cavity with a clean and dry environment, suitable for a significant upgrade of the facade life expectancy. On the other hand, an overly conservative calculation approach is generally against the optimization of the sustainability performance indices, determining an excessive use of materials and then impacting the overall life cycle cost under several perspectives. In particular, the objective of this second paper is to describe in a measurable way the permeability behavior of a CCF, defining statistical variability and nonlinearity effects measured during dedicated experimental testing. The permeability functions represent a fundamental input for the proposed assessment tool introduced by the first paper and it will be seen that an accurate quality control during the manufacturing can ensure the robustness of an optimized calculation approach, based on the load sharing mechanism between the skins. In the third paper it will be shown that the proposed tool, fed by the experimental permeability functions, appears adequate to predict the facade structural behavior under the superimposition of thermal loading, wind loading and dry air flow effects, when compared with measurements collected during outdoor testing on a facade specimen