The duct may be insulated, but how air tight is the connection between it and the register? Air leakages can be worse than uninsulated ducts.

HI, I would think about 4 or 5 Inches of loose insulation around your duct. Might cure your problem.

Good Luck Woodbutcher

I think DeWayne has correctly identified the issue as an air leakage issue between the attic and the conditioned living space. You need to air seal the interface to prevent warm, humid attic air from contacting the surface of the cool metal supply diffuser which is at or below the dewpoint of the attic air.

Did you tried removing the vent cover and looking for the source of the condensation? The connection between the insulated duct and the vent boot probably has no insulation covering. If that is the problem then all the ceiling vents will need to be inspected and insulated.

Classic problem. Moist attic air is reaching the cold metal surface of the register/ductwork, condensing and then coming through the gaps or soaking the drywall. Typically happens because there is some exposed metal surface on the register boot where the flex duct, etc. transitions to the boot/ceiling register. In a typical install, unless the installer pays close attention to detailing this transition area, some amount of cold boot surface is left exposed. In a sloppy install, it can be a lot of exposure. The solution is to cover the exposed metal with something that both insulates and is vapor impermeable. Canned spray foam is typically used to do this. Once foamed, the moist attic air "sees" a warmer foam surface and condensation doesn't occur. The biggest problem doing this is, IMO, is access; it can be a real pain in the rear getting to these register locations in some attics, if at all.