Under what conditions can the net free cross-ventilation area for attics be reduced to 1/300 in Climate Zones 6, 7, and 8?

The net free cross-ventilation area for attics can be reduced to 1/300 in Climate Zones 6, 7, and 8 if a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling and 40-50% of the required venting area is provided by upper ventilators located within 3 feet of the ridge.

In Climate Zones 6, 7, and 8, the net free cross-ventilation area for attics can be reduced from 1/150 to 1/300 under specific conditions. First, a Class I or II vapor retarder must be installed on the warm-in-winter side of the ceiling. Second, at least 40% and not more than 50% of the required venting area must be provided by ventilators located in the upper portion of the attic or rafter space. These upper ventilators should be positioned no more than 3 feet below the ridge or highest point of the space, with the remaining ventilation provided by eave or cornice vents. If structural members interfere with the installation of upper ventilators, they may be installed more than 3 feet below the ridge.