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2.2 Air 143 showing that the most air-tight layer buffers the greatest difference. Inclusion of such an ‘air barrier’ or ‘air retarder’ diminishes the total air permeance substantially and minimizes the air flux. Of course, air barriers must withstand the wind load. Is this flat assembly approach of practical relevance? Not really. The prerequisite of having one-dimensional flow already excludes stack with its pressure profile along a wall’s height. Also, wind is never uniform. Cracks, leaks and others also disrupt onedimensionality. 2.2.4.3 Two and three dimensions For isotropic open-porous materials, the calculation is a copy of two- and threedimensional heat conduction. Of course, mass conservation substitutes energy conservation – the sum of air flows from every neighbour to each central control volume zero: X Ga;i‡j ˆ 0 The algorithm thus becomes: X iˆl;m;n jˆ1 K´a ;i‡jPa;i‡j Pa;l;m;n X iˆl;m;n jˆ1 K´ a;i‡j ˆ 0 with K´a ;i‡j the air permeance between every neighbouring and each central control volume. The value within a material is: Ka ˆ kaA=a with A the contact surface with the adjoining volume and a the distance along the mesh between the centre of the neighbour and the control volume considered (Figure 2.8). l,m+1 a l-1,m l,m l+1,m a l,m-1 Fig. 2.8 Conservation of mass: sum of airflows from the adjoining to the central control volume zero


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