When insulating HVAC ducts, use sheet-coated fiberglass insulation with an R 6 value or higher. Use the type of metal adhesive tape recommended by the insulation manufacturer to seal and hold the insulation in place. Make sure that the surface of the insulation sheet is dust-free before applying the tape. HVAC ducts are usually insulated with fiberglass.
As a rule, flexible tube-shaped ducts usually have fiberglass covers. This type of insulation is coated with a shiny aluminum sheet that holds the fiberglass in place. They are particularly useful in attics and other small spaces that cannot be easily accessed. Insulating rolls and blocks are very common types of insulation for building houses and for completing insulation in attics.
Fiberglass is the most commonly used type of insulation rolls and blocks. That doesn't mean it's the best value for money. It is prone to leaving gaps around construction materials. Or it is simply crushed in place to fit, reducing its R value.