Abstract: Enclosed is an improved snap-on heat insulating jacket for ducting and method uniquely designed for simplicity and ease of assembly. Resilient heat insulating material is laminated to a flexible sheet plastic main body provided with interlocking snap-on seam members positioned, when closed, for concealment by an external guard flap coated on its inwardly facing surface with pressure sensitive adhesive thereby to embrace the ducting with a cover having a substantially continuous smooth outer surface free of foreign matter collecting crevices and the like.