Fibrous glass duct insulation means dry ductwork. When warm, humid air comes in contact with the surfaces of uninsulated cold air ductwork, water vapor will condense and result in dripping of liquid water from duct surfaces onto other building components. Installing insulation with sufficient R-value along with a low permeance vapor retarder will minimize, and in most cases prevent, condensation problems and consequent damage to building materials. Adding insulation is the best way to control surface condensation.