I am ready to add insulation to my attic. The attic is well ventilated (now) as I have installed fully vented soffits and my new roof has a ridge vent, plus I have gable vents with a fan on one end. I have the soffit baffles in. Problem-I only have about 4" blown in fiberglass with no vapor barrier. Do I remove the existing insulation and add a vapor barrier or just insulate over the existing? I live in the Baltimore-DC area.