Flexible insulation is the best to use for ductwork elbows and joints.
Properly placed ductwork insulation can help save you money on heating and cooling costs by improving the efficiency of your air conditioner and heater, because it will eliminate escaping cold and hot air. The insulation of ductwork elbows, because of the angles at which they bend, can be tricky. These steps will help you handle the task.
Clean the elbow junction you plan to insulate by either scrubbing it with a wire brush or rubbing it down with sandpaper.
Wipe the elbow clean and dry with old rag.
Cut a length of flexible aluminum insulation to fit the ductwork elbow.
Wrap the cut length of insulation around the elbow and seal it tightly with duct tape.