R-6 fiberglass duct wrap also works well, as long as it is is not compressed. R-6 ductwrap is approx. 2.5" thick. If it is applied tight to the duct, so that it "Looks Nice", it compresses the insulation and can reduce the R-factor to as little as R-2. Measure the circumference of the duct and add 14.5". Cut that much duct wrap and wrap it around the duct with a 2" overlap. Staple and tape the seams. Use only tape rated UL-181 B-FX. The 14.5" add-on stretch out is the amount recommended by NAIMA (North American Insulation Manufacturers Association.) for 2.5" thick duct wrap installed on round duct.. Different thicknesses have different recommended stretchouts. For 2.5" thick wrap on rectangular duct the recommended add on is 12.5".