Condensation in Buildings - Australian Building Codes Board

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Information Handbook: Condensation in Buildings Water Vapour Control under Floors The suspended timber floor shown in Figure 5.5 offers intentionally low resistance to the diffusion of water vapour into the ventilated sub-floor space to allow higher indoor water vapour levels to be dissipated in the ventilating air stream. Where this practice has been applied to air conditioned houses in the United States, problems have emerged with inward vapour drive from the sub-floor space under summer conditions (Lstiburek 2008). The migrating water vapour can condense on surfaces with high vapour resistance such as vinyl flooring or the bases of furniture and cupboards in direct contact with the floor. In the high relative humidity created within the fibrous insulation, timber framing is at risk of mould and decay. Attempting to minimise inward diffusion by installing foil faced foam insulating board as a vapour control layer under the joists (Figure 5.18) requires an air and vapour tight installation to be achieved under difficult working conditions. If successfully sealed, the foam insulating layer could be at risk of trapping any liquid leaking through the floor. An alternative would be to encase the floor framing and underside of flooring in high density sprayed foam (minimum 32kg/m³) at least 75 mm thick. Figure 5.18 – Suspended timber framed floor with summertime risk of inward vapour drive DRAFT HANDBOOK Australian Building Codes Board Page 73
Information Handbook: Condensation in Buildings Vapour Permeable Materials There is considerable potential for confusion in the Australian market for anyone seeking membranes or other building materials with verified levels of water vapour permeance or water vapour resistance. The confusion could start with the two terms but they are simply measures of how ready or reluctant a particular material is to allow the passage of water vapour. A performance value for one is the reciprocal of the other. AS/NZS 4200.1:1994 identifies three classifications of water vapour barriers, ranked High, Medium or Low according to their resistance. Low resistance under AS/NZS 4200.1:1994, however, does not necessarily mean high permeance because of the range of resistances accepted as Low. Some products are marketed as “breather” membranes, implying high permeability, but might not state performance values for either vapour resistance or vapour permeance. Figure 5.19 shows permeance values for a range of building materials and membranes, grouped within the resistance classifications of AS/NZS 4200.1:1994. Comparing the performance of the “breather” membranes suggests the possibility of unexpected outcomes if materials critical to an intended water vapour control strategy are specified or supplied on the basis of generic labels rather than specific performance requirements. Figure 5.19 – Water vapour permeance of some building materials and permeable membranes HIGH water vapour resistance classification (AS/NZS 4200.1:1994) steel glass 'breather' 1 MEDIUM water vapour resistance classification (AS/NZS 4200.1:1994) polyethylene aluminium foil 'breather' 2 'breather' 3 vinyl wallpaper paints 'breather' 4 LOW water vapour resistance classification (AS/NZS 4200.1:1994) plasterboard brick (110 mm) polyurethane (20 mm) oriented strand board sheathing plywood fibre cement sheeting mineral wool (100 mm) 'breather 5' urea formaldehyde (20 mm) low resistance (LR) underlay DRAFT HANDBOOK 0 2 4 6 8 10 12 14 Permeance (rate of water vapour transmission – μg/N.s) Australian Building Codes Board Page 74
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Condensation in Buildings - Australian Building Codes Board

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