How to Stop Condensation in the Attic A/C Unit Ducts

Seal the ducts using mastic sealant or metal tape. The vast majority of efficiency losses through air conditioning ducts occurs because the ducts have not been properly sealed. Never use duct tape to seal ducts, as it is not nearly as effective as other materials such as mastic sealant or metal tape. Apply the sealant to all openings in the ducts exactly according to the manufacturer's directions.

Apply insulation to the outside of the attic air conditioning ducts. Having sealed openings in the duct work, it is necessary to help mitigate the temperature differences between the inside of the ducts and out that cause condensation to form. Rigid fiber board insulation is ideal for achieving this, insulated to an R-value of R-4 to R-11, depending on climate.

Keep air conditioning duct vents clear of any blockage. Inefficiencies in air conditioning duct work that cause condensation to form on the ducts themselves are very often caused by physical obstructions in front of air conditioning vents that force air back into the ducts, reducing the overall efficiency of the system. Inspect each air conditioning vent in the home and remove any obstructions that may block the flow of air conditioned air through them.