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You should have one square foot of vent area for every 150 square feet of attic floor space. The vent manufacturer will list the vent area on the package as the FVA (free vent area). The vent area must also be split between high and low vents. If a vapor barrier is present, then the requirement changes to one square foot for every 300 square feet. Shingle manufacturers require that attics have adequate ventilation; otherwise their product warranty may be voided.