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2.2 Air 137 Fig. 2.3 Air-open layers: tiled roof covers, slated roof covers and a lathed ceiling mortar and bricks in masonry, nail holes, leaky joint fillers and loose connections. Intended leaks include the electricity boxes in walls, light spots in a ceiling, trickle vents above windows, and so on (Figure 2.4). Many outer wall, party wall and roof solutions have cavities and voids included. In an open porous material, air displacement follows Poiseuille’s law of proportionality between air flux and driving force, in this case being the air pressure gradient: ga ˆ ka grad Pa (2.16) The factor ka is called the air permeability, with units of seconds. The minus sign indicates that air moves from points at higher to points at lower air pressure. The permeability is a scalar for isotropic materials, and a tensor with three main directions and a value differing per direction for anisotropic materials. Its value increases together with the open porosity and the number of macro-pores. For air permeable layers, leaks, cracks, joints, cavities and openings, the equations are: ψa Air permeable ψa layers (per m2): Ga ˆ KaΔPa, with Ga in kg/(m2.s), Ka in s/m. Joints; cavities …per m† : Ga ˆ KΔPa; with Ga in kg=…m:s†;Kin s: Leaks; openings …per unit† : Ga ˆ Kχa ΔPa; with Ga in kg=s; Kχa in m:s: (2.17) with Ka, Kψa and Kχa the air permeances and ΔPa the air pressure differentials. Because the flow is not necessarily laminar, most permeances depend on the pressure Fig. 2.4 Spotlight and trickle vent as intended leaks


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