9.19.1.2. Vent Requirements
This Article requires sufficient #vent area to transfer to the exterior any #moisture that may collect in roof spaces before it can create problems. Vents not only reduce the amount of winter moisture accumulation, but also allow the roof or #attic space to dry out more quickly in the spring when the temperature rises to a level that might initiate wood decay. In low-sloped roofs, the roof #sheathing is closer to the source of moisture leakage from the space below, and #condensation and frost build-up can occur before the moisture escapes through the roof vents. Therefore, greater vent area is required for such roofs, both to reduce moisture build-up and to hasten subsequent spring drying.
The distribution of vents on opposite sides of a roof is intended to encourage #cross-ventilation, while top and bottom distribution is intended to take advantage of convection currents to move heated air to the highest part of the roof to escape. If there is no common #attic space (e.g., flat roofs, cathedral ceilings) and there is no interconnection between joist spaces, the only way the roof can be effectively vented is by venting each #joist space. If the vents allow the entry of snow or rainwater, this will add to the moisture load and the risk of problems previously noted.
Quoted from Illustrated Users' Guide --NBC2015
#roofspace #roofvent #crossventilation
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