Calculating the thickness of the insulation needs a complicated heat transfer calculation. The insulation R value can be R-6 or R-8 for ductwork. Usually 1" thick duct insulation is sufficient for the indoor ductwork and 2" thick for outdoor ductwork.

See the link below the sample guideline for heat transfer calculation trough ductwork and the required insulation thickness. However after calculation the result would be ended up to what I explained above.

http://www.isover.co.uk/sites/isover.uk/files/assets/documents/hvac-handbook.pdf