Tempo:
assemblies. These processes act on the building envelope in different ways and with different intensities. The assemblies and materials that comprise the building envelope are selected and arranged to effectively control these four processes:
Water penetration control prevents water from entering the interior space or building envelope assemblies and causing premature deterioration of materials. This is the most important function because water potentially causes the most rapid and destructive deterioration of the building envelope. Air leakage control through the materials, interfaces and joints of the building envelope provides occupant comfort, minimizes heating and cooling energy use; and limits the potential for condensation. Uncontrolled air leakage allows moisture to move through the building envelope, which can also result in severe deterioration of the building envelope assemblies. Vapour diffusion control through the materials that comprise the building envelope limits the potential for condensation. Water vapour diffusion through the building envelope is a very slow and uniform physical process. As a result, the accumulation of condensation-related moisture occurs slowly and is less likely to lead to damage. Heat flow control through the materials that comprise the building envelope provides occupant comfort, minimizes heating and cooling energy use; and limits the potential for condensation.
The designer must choose appropriate assemblies of materials and develop appropriate interface and penetration details to address all of these control functions. The following sections describe how design and construction of wood-frame houses can control water penetration, air leakage, vapour diffusion and heat loss.
WATER PENETRATION CONTROL The building envelope for a wood-frame house controls exterior precipitation (usually water from rain, snow and ice melt) by deflecting moisture away from a wall, draining any moisture that penetrates into the building envelope back to the exterior, and allowing any accumulated moisture to dry over time. See Chapters 12 to 15 for specific construction details. All walls are required to have a primary and a secondary line of defence against rain penetration. The first plane of protection is typically the cladding (for example, siding, masonry or stucco). It includes accessories such as trim or caps that are part of the cladding system. The second plane of protection ordinarily consists of a sheathing membrane or insulating sheathing, flashing, sealants and other materials, and is intended to:
intercept any water that has penetrated the first plane; and drain those relatively small quantities of incidental water to the exterior.
Both the first and second planes of protection must provide adequate protection of the wall assembly where penetrations have been made for windows, doors, ventilation ducts, piping, wiring and electrical outlets; and at the interface with other wall assemblies. These penetrations and interfaces at openings are where most wall assemblies develop problems over their lifetime. Deflection: components and features of the building envelope (such as overhangs) that limit the exposure of assemblies and details to water running along surfaces of the building envelope and divert it from the assembly (shingle lapping materials and drip edges and flashings).
Water penetration control prevents water from entering the interior space or building envelope assemblies and causing premature deterioration of materials. This is the most important function because water potentially causes the most rapid and destructive deterioration of the building envelope. Air leakage control through the materials, interfaces and joints of the building envelope provides occupant comfort, minimizes heating and cooling energy use; and limits the potential for condensation. Uncontrolled air leakage allows moisture to move through the building envelope, which can also result in severe deterioration of the building envelope assemblies. Vapour diffusion control through the materials that comprise the building envelope limits the potential for condensation. Water vapour diffusion through the building envelope is a very slow and uniform physical process. As a result, the accumulation of condensation-related moisture occurs slowly and is less likely to lead to damage. Heat flow control through the materials that comprise the building envelope provides occupant comfort, minimizes heating and cooling energy use; and limits the potential for condensation.
The designer must choose appropriate assemblies of materials and develop appropriate interface and penetration details to address all of these control functions. The following sections describe how design and construction of wood-frame houses can control water penetration, air leakage, vapour diffusion and heat loss.
WATER PENETRATION CONTROL The building envelope for a wood-frame house controls exterior precipitation (usually water from rain, snow and ice melt) by deflecting moisture away from a wall, draining any moisture that penetrates into the building envelope back to the exterior, and allowing any accumulated moisture to dry over time. See Chapters 12 to 15 for specific construction details. All walls are required to have a primary and a secondary line of defence against rain penetration. The first plane of protection is typically the cladding (for example, siding, masonry or stucco). It includes accessories such as trim or caps that are part of the cladding system. The second plane of protection ordinarily consists of a sheathing membrane or insulating sheathing, flashing, sealants and other materials, and is intended to:
intercept any water that has penetrated the first plane; and drain those relatively small quantities of incidental water to the exterior.
Both the first and second planes of protection must provide adequate protection of the wall assembly where penetrations have been made for windows, doors, ventilation ducts, piping, wiring and electrical outlets; and at the interface with other wall assemblies. These penetrations and interfaces at openings are where most wall assemblies develop problems over their lifetime. Deflection: components and features of the building envelope (such as overhangs) that limit the exposure of assemblies and details to water running along surfaces of the building envelope and divert it from the assembly (shingle lapping materials and drip edges and flashings).
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