Check your ducts for air leaks. First look for sections that should be joined but have separated and then look for obvious holes.
If you use duct tape to repair and seal your ducts, look for tape with the Underwriters Laboratories (UL) logo to avoid tape that degrades, cracks, and loses its bond with age.
Remember that insulating ducts in the basement will make the basement colder. If both the ducts and the basement walls are uninsulated, consider insulating both.
If your basement has been converted to a living area, install both supply and return registers in the basement rooms.
Be sure a well-sealed vapor barrier exists on the outside of the insulation on cooling ducts to prevent moisture buildup.
Get a professional to help you insulate and repair all ducts