How to Build a Fiberglass Duct

Position the duct board, foil side down on a clean, flat surface. To construct a 2-by-4 foot duct, cut a sheet of duct board to a length of 8 feet. Place your tape measure on the thickest edge of the board on the staple flap side and draw a centerline every 2 feet. Do not include the staple flap when making these measurements.

Cut the corner grooves using the shiplap corner tool. Start with the first centerline closest to the staple flap and carefully drag the tool along the line. The tool has a sharp blade that cuts an applicable size groove into the board allowing it to fold together correctly. Repeat this step for the remaining centerlines.

Cut the closure joint on the end of the duct board opposite the staple flap using the corner shiplap tool. Use the edge of the duct board as a guide and carefully drag the tool along the edge of the board. The corner shiplap tool cuts the applicable size corner joint rabbet that will join with the staple flap side. It works in the same manner as the corner groove tool.

Remove the fiberglass insulation from the staple flap by carefully cutting a groove against the beginning edge of the duct board without cutting through the foil backing. This allows the staple flap to overlap the closure joint.

Fold the duct board sections carefully to ensure that the shiplap corners fold correctly on center. Join the corner rabbet cut with the flush edge at the closure joint.

Fold the corner flap over the corner of the duct and staple with a staple gun to hold it in place. Space the staples about 2 inches apart the length of the duct.

Install metal foil A/C duct tape to secure and seal the corner joint. Center a length of tape on the corner and press into place. Overlap each side of the first row of tape with an additional piece of foil tape. Use a tape squeegee to remove any air bubbles or creases.