How to Cut and Join Duct Board

Starting at the left edge of the board, measure and mark the board's core material with the three fold lines for the duct's panels, a line for the stapling flap, and a cut-off line on the right edge.

Cut a rabbet joint into the inside left edge with a rabbet shiplap tool. A rabbet is a recessed cut that allows the right edge and the left edge to form a tight seal. To cut the rabbet joint, carefully drag the rabbet shiplap tool along the edge of the board.

Cut grooves in the lines you marked in Step 1, using a modified corner shiplap tool. The shiplap tool is used to cut grooves into the lines, marking the corner folds. As you cut from the left, be sure to make a right shiplap groove, then a left groove, and then another right groove.

Create the stapling flap by cutting through the insulation with a utility knife and stripping the insulation from the board's facing material. Be careful not to cut into the facing.

Fold the the duct board to form the section of duct, ensuring that the rabbet created on the left edge in Step 2 seats properly with the closure joint on the right edge.

Hold the duct section about 30 degrees beyond square and staple the joint every 2 inches. Place each staple about 1/2 inch from the edge of the stapling flap.

Seal the joint with pressure-sensitive aluminum foil tape, using a squeegee or similar burnishing tool to rub the tape and create a strong bond. For 1-inch duct board, use 2 1/2-inch-wide tape. For 1 1/2- or 2-inch duct board, use 3-inch-wide tape.