My contractor suggested installing a ridge vent on my roof and I already have two single static vents. Do I really need a ridge vent?

NRCA suggests the amount of attic ventilation be balanced between the eaves and ridge. The intent of a balanced ventilation system is to provide nearly equivalent amounts of ventilation area at the eave/soffit and at or near the ridge. For a balanced ventilation system to function properly, approximately one-half of the ventilation area must be at or near the ridge.