Supply Duct Insulation

[Daniel Hsieh]

one of the items in the inspection report for a house I am selling is

IRC 2009 N1103.2.1 Insulation. Supply ducts in attics shall be insulated to a minimum of R-8. All other ducts shall be insulated to a minimum of R-6. There are R-6 ducts installed in the attic, information should be obtained from the seller as to the specific energy rating system for the home. 

The buyer keep insisting me to change it to R8 insulation. I can't reach the builder to find out the energy rating of the home. Am I doomed to have to replace it? What kind of rebuttal or after-market certification I can get to justify that R6 insulation in my house is in deed  enough. The buyer is a construction manager by profession so obviously I need to give him something concrete to convince him otherwise. Thanks....any comments are appreciated....

Daniel