How much insulation do HVAC ducts in the attic need?

Supply and return ducts in unconditioned attics must be insulated to R-8, and other ducts outside conditioned space to R-6 (IRC N1103.2.1).

Ducts running through an unconditioned attic must be insulated to at least R-8. Ducts located in other unconditioned spaces (crawlspaces, unheated basements, exterior wall cavities) require at least R-6. Ducts located completely inside the conditioned space don't need insulation.

This is section N1103.2.1 in the 2012 IRC, renumbered to N1103.3.1 in the 2015 IRC. The 2015 edition adds a duct-diameter distinction (larger ducts R-8, smaller ducts a reduced value in attics), so check your adopted edition for the fine print.

Separately from insulation, ducts must be sealed at joints and seams, and the duct system may have to pass its own leakage test.