A. Henry Spies responds: The amount of ridge vent needed is not related to the roof area, it is related to the living space below the roof. Most codes require roof ventilation having a minimum total net free ventilating area of 1/150 of the ceiling area of the living space beneath it. This can be reduced to 1/300 of the area if at least 50% and less than 80% of the required ventilation area is provided by ventilators located in the upper portion of the space to be ventilated, at least 3 feet above eaves or cornice vents, with the balance of the ventilation provided by eaves and cornice vents. As an alternative, the net free cross-ventilation area can be reduced to 1/300 when a vapor barrier having a transmission rate not exceeding 1 perm is installed on the warm side of the ceiling.
The net free ventilation area of a ridge vent is determined by the manufacturer. Wind tunnel tests indicate the effective net free area of ventilation provided by most ridge vents is typically half that indicated by the manufacturer. There is no problem with overventing the attic area, so the best course is to provide as much ridge vent as can...
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