Tempo:
the air space in masonry cavity walls allowing moisture to drain to the exterior.
In order to be effective, a capillary break must be provided over the full height and width of a wall, except where openings (such as windows and doors), service penetrations (such as pipes, ducts and electrical outlets) or flashings are present. Where furring is used to provide the gap, it must not cover more than 20 per cent of the wall area. A screen should be installed at the bottom of the capillary break or air space at foundations, windows and doors to prevent insects from getting in. The top of the capillary break should not vent into the attic space as this can lead to condensation on surfaces in the attic. Although blocking at the top of the capillary break restricts ventilation, it does not impede drainage, which is the more important aspect of water penetration control for rainscreen construction. In conventional wood-frame construction, sheathing membranes are applied over the OSB or plywood sheathing in horizontal strips, shingle-lapped at the joints and stapled. When rigid or semi-rigid insulation is applied over the exterior of wall sheathing, the sheathing membrane may be applied over the outside face of the insulation (when the outboard insulation type must be protected from wetting) and fastened through to the frame wall. Alternatively, the sheathing membrane may be applied over the wall sheathing and on the inside face of the insulation (when the outboard insulation type does not need to be protected from wetting). Some rigid insulation products such as extruded polystyrene are also relatively vapour impermeable. If used, care must be taken in the selection of cladding material and detailing to ensure that no significant amounts of water reach the interface between the extruded polystyrene insulation and the vapour permeable sheathing membrane. This risk may dictate the use of insulation products with high vapour permeability and low water retention, such as mineral fibre.
The sheathing membrane should be installed so that it intercepts any moisture that gets behind the cladding (the first plane of protection) and directs it outward over flashings, at openings, at changes in cladding materials and at the base of walls. The sheathing membrane must be permeable enough to permit water vapour to diffuse from a wall assembly to the exterior. One layer of sheathing membrane is generally used and may be applied horizontally or vertically with 100 mm (4 in.) or greater laps at the joints. A second layer may be required for added moisture protection. Common breather-type sheathing membranes on walls are asphalt-saturated felt, building paper and spun-bonded polyolefin (housewrap). Vapour permeable liquid membranes and self-adhered sheet products are also used for sheathing membranes. Some sheathing membrane materials (such as spun-bonded polyolefin) can also be utilized as part of the air barrier system, where they are required to be structurally supported. Self-adhered vapour permeable sheet membranes are desirable in these situations because the adhesion helps to provide the necessary structural support. Where wall sheathing is not used, two layers of sheathing membrane are needed unless large panel siding such as plywood is used. Apply both layers vertically with 100 mm (4 in.) or greater laps at the studs. Staple both layers to the framing members, the top layer with staples spaced 150 mm (6 in.) or less along the edges of the sheet to hold it securely in place. See Chapter 10 for more information.
Roofs On pitched roofs with slopes where the rise and run are equal to or greater than 1:6, asphalt-saturated sheathing paper is applied over the roof sheathing to provide a secondary plane of protection to water that may penetrate beyond the primary plane of protection—the shingles, tiles or metal roofing.
In order to be effective, a capillary break must be provided over the full height and width of a wall, except where openings (such as windows and doors), service penetrations (such as pipes, ducts and electrical outlets) or flashings are present. Where furring is used to provide the gap, it must not cover more than 20 per cent of the wall area. A screen should be installed at the bottom of the capillary break or air space at foundations, windows and doors to prevent insects from getting in. The top of the capillary break should not vent into the attic space as this can lead to condensation on surfaces in the attic. Although blocking at the top of the capillary break restricts ventilation, it does not impede drainage, which is the more important aspect of water penetration control for rainscreen construction. In conventional wood-frame construction, sheathing membranes are applied over the OSB or plywood sheathing in horizontal strips, shingle-lapped at the joints and stapled. When rigid or semi-rigid insulation is applied over the exterior of wall sheathing, the sheathing membrane may be applied over the outside face of the insulation (when the outboard insulation type must be protected from wetting) and fastened through to the frame wall. Alternatively, the sheathing membrane may be applied over the wall sheathing and on the inside face of the insulation (when the outboard insulation type does not need to be protected from wetting). Some rigid insulation products such as extruded polystyrene are also relatively vapour impermeable. If used, care must be taken in the selection of cladding material and detailing to ensure that no significant amounts of water reach the interface between the extruded polystyrene insulation and the vapour permeable sheathing membrane. This risk may dictate the use of insulation products with high vapour permeability and low water retention, such as mineral fibre.
The sheathing membrane should be installed so that it intercepts any moisture that gets behind the cladding (the first plane of protection) and directs it outward over flashings, at openings, at changes in cladding materials and at the base of walls. The sheathing membrane must be permeable enough to permit water vapour to diffuse from a wall assembly to the exterior. One layer of sheathing membrane is generally used and may be applied horizontally or vertically with 100 mm (4 in.) or greater laps at the joints. A second layer may be required for added moisture protection. Common breather-type sheathing membranes on walls are asphalt-saturated felt, building paper and spun-bonded polyolefin (housewrap). Vapour permeable liquid membranes and self-adhered sheet products are also used for sheathing membranes. Some sheathing membrane materials (such as spun-bonded polyolefin) can also be utilized as part of the air barrier system, where they are required to be structurally supported. Self-adhered vapour permeable sheet membranes are desirable in these situations because the adhesion helps to provide the necessary structural support. Where wall sheathing is not used, two layers of sheathing membrane are needed unless large panel siding such as plywood is used. Apply both layers vertically with 100 mm (4 in.) or greater laps at the studs. Staple both layers to the framing members, the top layer with staples spaced 150 mm (6 in.) or less along the edges of the sheet to hold it securely in place. See Chapter 10 for more information.
Roofs On pitched roofs with slopes where the rise and run are equal to or greater than 1:6, asphalt-saturated sheathing paper is applied over the roof sheathing to provide a secondary plane of protection to water that may penetrate beyond the primary plane of protection—the shingles, tiles or metal roofing.
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