R6 or R8 Bubble Wrap
"Reflectix ® Duct Insulation is an alternative to other types of fibrous insulation products. Ease of handling and quick installation make Reflectix ® the first choice when selecting insulation for round or rectangular ductwork in HVAC systems. The product provides an R-6.0 or R-8.0 when installed with a 0.75” air gap between the duct and the insulation. There is a choice of three spacer configurations that create the required air space. "