Before you insulate
Prior to insulating, you need to seal the ducts. This can be achieved with metal foil tape or duct mastic. DO NOT USE DUCT TAPE, it will not last. When using the foil tape, the ducts must be free of dust and dirt or the tape will not stick. The mastic comes in a gallon bucket and can be applied with a small stiff paint brush. It can also come in tubes and be applied with a caulking gun. Cover all the duct joints, takeoffs, and connections as well as any small holes you find. This will not only save you energy dollars. By sealing the return air ducts, you can stop the hvac system from drawing in pollutants from the unconditioned areas. This can improve the home's overall indoor air quality.