Abstract: Thermal insulating panels for application to HVAC rectangular ductwork each comprise a first layer of solid insulation with a second layer of solid insulation laminated to the first layer. An outer metal layer of embossed aluminum is laminated to the second layer of each panel. These panels are secured to the outside of the rectangular ductwork with at least four interlocking panels, one on each side thereof joined together at the four corners of the ductwork. The outside metal layer of embossed aluminum of at least some of the panels includes right angle flanges along the sides thereof for attachment to an adjacent panel. A number of panels are arranged end-to-end in a series along each side of the rectangular duct, and an overlapping joint is provided between adjacent panels in the series.