The primary causes of condensation on ductwork in unconditioned crawlspaces and attics is missing or otherwise inadequate duct insulation, no or poor vapor barrier on the ducts and/or too much moisture in the ambient air. If the above are addressed properly this usually eliminates or minimizes the problem greatly. Sealing the crawlspace is generally recommended as well, and a vapor barrier on the crawl space floor or ground to decrease moisture absorption into the air is a good idea. A free standing dehumidifier should also help. Aeroseal may help to a lesser degree if duct air leaks, beneath insulation are causing the duct surface temperature to reach dew point, which is the temperature when condensation begins to form. Over time this condensation will degrade the insulation’s R value and its ability to prevent the duct temperature from reaching dewpoint.