Generally, you will not need insulation for heating and air-conditioning ducts that are enclosed in already well-insulated portions of your house. But, if the ducts for either your heating or your air-conditioning system run above the ceiling or below the floors, they should be insulated.

Whether ducts are in the attic or beneath the floor, insulation is installed in pretty much the same way using special duct insulation:

• Seal seams with metal duct tape.
• Cut the insulation so it's long enough to wrap around the duct with a two-inch overlap.
• Wrap the insulation around the duct with the vapor barrier on the outside, faced away from the ducts.
• If your ducts already have some insulation, check to see if any moisture has collected in it. If so, replace it with new insulation.
  Note: some duct systems have insulation on the inside that cannot be seen. Check to make sure you do not have this type of duct system before adding additional insulation.