Shouldn't the plenum be insulated instead of bare sheetmetal even if it's in the crawspace?

[Anonymous]

Ducts are required too be insulated in all unconditioned spaces. International Mechanical Code section  603 and 604. In Addition they are required to be sealed and if insulated on the outside of the ducts, they are required to have a vapor retarter covering. Duct wrap 2 inches and sealed with UL181A-P tape. Are you sure the ducts are not insulated on the inside?

[owlnevada]

If it should be insulated, what type is used and how to fasten it to the sheetmetal? Is the building code allowing that as standard contractor specs to leave it bare?