Here is a picture of an 18 inch duct that is fully insulated to R38 or more. It has two “thermally broken” damper assemblies buried inside cellulose insulation. The cellulose in the picture is rated R13 to R60. You will note that the R rating is determined by the thickness – or, in this case, the diameter of the fence around the ducts. In the picture the fence is 10 inches wide. It needs to be 14 inches for you to get R38 from it. And note that if you make it even wider it can approach R60.

R38 Ceiling Adapter for 18 inch duct