Tempo:
Pitched roofs with slopes less than 1:6 require a waterproof membrane, usually consisting of one or more layers of bituminous membrane with joints shingle-lapped and welded, self-adhering “peel-and-stick”membrane, reinforced liquid applied membranes, or an EPDM (ethylene propylene diene monomer) rubber membrane. The lower pitch of these roofs does not shed or drain water as readily as a steeper pitch, and therefore requires a more robust and completely waterproof membrane.
AIR LEAKAGE CONTROL Water vapour can be driven through the building envelope by air movement (which can be controlled by an air barrier system) and vapour diffusion (which can be controlled by the use of low-vapour-permeable materials placed at appropriate locations relative to the insulating materials). This section describes how the air barrier in the building envelope controls air movement. Vapour diffusion is discussed in the “Vapour Diffusion Control” section on page 44. Wood-frame houses should be constructed as airtight as possible. In addition to protecting the building envelope, airtightness promotes energy efficiency, eliminates thermal comfort issues related to drafts, allows for better control of natural and mechanical ventilation, reduces the transmission of outdoor noise and controls odours. Air leakage through the building envelope can also result in water vapour condensing on cold surfaces, and this moisture can lead to deterioration of materials within the building envelope if allowed to accumulate for sufficient time. Airtightness is cumulative—good workmanship in the construction of each element of the entire building envelope is essential to its proper performance. Air leakage through the building envelope occurs as a result of differences in air pressure from inside to outside the building caused by
the operation of fans or the action of wind. When the air pressure is greater inside than outside, air will flow outwards through any holes or cracks in the building envelope, carrying with it any water vapour it contains. This is called “exfiltration.” The reverse, called “infiltration,” is true when the air pressure outside is greater than inside, and air will flow in through holes and cracks.
Air Barrier System The NBC stipulates that the building envelope must be designed and constructed with a continuous air barrier system comprised of air barrier assemblies to control air leakage into or out of the conditioned space. Air leakage control requires a continuous air barrier system carried through the basement floor and walls, above-grade exterior walls and ceilings adjacent to roofs. Air leakage control within building envelope assemblies can be accomplished by incorporating materials with low air permeability, which are fairly common in house construction. However, joints and penetrations must also be sealed to stop air leakage, and the air barrier must be supported to prevent deformation or tearing. Polyethylene sheets are commonly used as an air barrier in wood-frame construction and are relatively easy to secure, seal and support on wood framing. In addition to polyethylene sheets, other materials such as spun-bonded polyolefin, gypsum board, sealants, gaskets, spray-applied foam, rigid insulation, concrete, plywood or OSB sheathing, metal and glass can be used as part of the air barrier. All joints, connections and penetrations must be sealed using tapes, gaskets, foams, sealants and membranes in order for the building envelope to be airtight.
Location of the Air Barrier System Because air movement through holes and cracks in the building envelope plays a much greater role in transporting water vapour than vapour
AIR LEAKAGE CONTROL Water vapour can be driven through the building envelope by air movement (which can be controlled by an air barrier system) and vapour diffusion (which can be controlled by the use of low-vapour-permeable materials placed at appropriate locations relative to the insulating materials). This section describes how the air barrier in the building envelope controls air movement. Vapour diffusion is discussed in the “Vapour Diffusion Control” section on page 44. Wood-frame houses should be constructed as airtight as possible. In addition to protecting the building envelope, airtightness promotes energy efficiency, eliminates thermal comfort issues related to drafts, allows for better control of natural and mechanical ventilation, reduces the transmission of outdoor noise and controls odours. Air leakage through the building envelope can also result in water vapour condensing on cold surfaces, and this moisture can lead to deterioration of materials within the building envelope if allowed to accumulate for sufficient time. Airtightness is cumulative—good workmanship in the construction of each element of the entire building envelope is essential to its proper performance. Air leakage through the building envelope occurs as a result of differences in air pressure from inside to outside the building caused by
the operation of fans or the action of wind. When the air pressure is greater inside than outside, air will flow outwards through any holes or cracks in the building envelope, carrying with it any water vapour it contains. This is called “exfiltration.” The reverse, called “infiltration,” is true when the air pressure outside is greater than inside, and air will flow in through holes and cracks.
Air Barrier System The NBC stipulates that the building envelope must be designed and constructed with a continuous air barrier system comprised of air barrier assemblies to control air leakage into or out of the conditioned space. Air leakage control requires a continuous air barrier system carried through the basement floor and walls, above-grade exterior walls and ceilings adjacent to roofs. Air leakage control within building envelope assemblies can be accomplished by incorporating materials with low air permeability, which are fairly common in house construction. However, joints and penetrations must also be sealed to stop air leakage, and the air barrier must be supported to prevent deformation or tearing. Polyethylene sheets are commonly used as an air barrier in wood-frame construction and are relatively easy to secure, seal and support on wood framing. In addition to polyethylene sheets, other materials such as spun-bonded polyolefin, gypsum board, sealants, gaskets, spray-applied foam, rigid insulation, concrete, plywood or OSB sheathing, metal and glass can be used as part of the air barrier. All joints, connections and penetrations must be sealed using tapes, gaskets, foams, sealants and membranes in order for the building envelope to be airtight.
Location of the Air Barrier System Because air movement through holes and cracks in the building envelope plays a much greater role in transporting water vapour than vapour
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