Roof space ventilation
Always provide adequate ventilation of the roof space above the insulation. Accumulated water vapour in attic spaces and under low-slope roofs could condense during cold weather in sufficient quantity to cause damage. Since most types of roof membranes are highly resistant to vapour transmission, vapour does not pass through them and ventilation is needed to remove vapour that enters the roof space.
During cold weather, heat lost through the ceiling insulation combined with sun exposure may provide enough heat to melt the snow on a roof but not on the projecting eaves. Water from the melting snow can freeze and form ice dams at the eavestrough and roof overhang, which are exposed to outdoor temperatures on all sides and are therefore colder than the ceiling. This may cause water to back up at the eaves, penetrate through the roof and leak into the walls and ceilings (see Figure 100). Adequate airtightening in combination with insulation and ventilation beneath the roof sheathing will keep attic temperatures low during winter and help prevent snow on the roof from melting. A space above the insulation between the exterior walls and the underside of the roof sheathing allows ventilation from the eaves to the attic space. When no air ventilation baffles are used, the clearance must be at least 63 mm (21⁄2 in.). If baffles are used, the clearance can be reduced to 25 mm (1 in.).
Roofs with slopes of 1:6 and greater are required to have an unobstructed vent area at least 1⁄300 of the insulated ceiling area, measured on a horizontal plane. For roofs with slopes less than 1:6, the area must be at least 1⁄150 of the ceiling area. Roof-, gable- or eave-type vents can be used (see Figures 92 and 93). Vents must be uniformly distributed with at least 25 per cent of the vent area located at the bottom of the space and at least 25 per cent at the top.
If batt insulation is placed between the roof-ceiling joists in a low-slope roof, a ventilated space of at least 63 mm (21⁄2 in.) must be provided between the top of the insulation and the underside of the roof sheathing. This can be achieved by placing 38 × 64 mm (2 × 3 in. nominal) cross-purlins over the top of, and at right angles to, the roof-ceiling joists. The cross-purlins may be shimmed to provide the required roof slope (see Figure 25 on p. 57). If rigid insulation is installed on top of the roof sheathing (and below the roof covering), the space above the ceiling is not ventilated.
Vents must not allow the entry of rain, snow or insects. If a vent is a potential source of moisture entry, a baffle is required to reduce air flow. Corrosion-resistant metal or plastic materials should be used for vents and to screen ventilator openings.
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