A fastening channel configured for use in insulating uninsulated ductwork is provided. The fastening channel includes a plurality of members forming one or more cavities. The cavities are configured to receive sections of an insulation envelope. The insulation envelope is formed from a duct board. The duct board is formed from a thermoplastic polymer sheet, a plurality of facing sheets and a layer of foam insulation. A plurality of angled splines extends from the plurality of members and form a plurality of clamps. The clamps are configured to engage one of the facing sheets such as the maintain the insulation envelope in place.