An intermediate insulating product (20) is disclosed comprising a planar insulating layer (11) having a top side in to which is formed a plurality of parallel channels (14) having cross-sections with tapered sides. A vapor proof layer (18) is applied to the top surface after the channels (14) have been formed such that the vapor proof layer (18) bridges the plurality of parallel channels (14). The intermediate insulating product (20) is then bent with mechanical manipulation in regions adjacent the bottom of the channels (14) thereby causing the channels (14) to close to form a non-planar, derivative insulated ductwork product (30) and the vapor proof layer (18) forms a vapor proof inner lining to the derivative insulated ductwork product (30).