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Bibliographic info:
Building Practice Note No 57. Ottawa:National Research Council Canada, Division of Building Research, October 1985.

Roof space ventilation is important in warm weather to dry out moisture that may have accumulated in the roof space. It is important to reduce the movement of moisture from the living areas to the roof space and to ventilate the latter by means of vents distributed between the upper and lower parts of theroof to take advantage of both wind action and stack action. To reduce the risk of problems resulting from condensation or hoarfrost in the roof space,it is recommended that the relative humidity inside the building be kept at a low level, and that the ceiling be made as airtight as possible. The total netvent area should be at least equal to 1/300th or 1/150th of the insulated ceiling area (depending on the type of roof space), and be distributed between the upper and lower parts of the roof. Air circulation should be unrestricted by insulation material or the roof framing.