How to Make Fiberglass Ducts

Measure and mark the size of your finished duct on the ductboard. Using your needed ductwork dimensions, determine the cumulative widths of all four sides of your ductwork by adding all four-side dimensions together. Mark this dimension on your ductboard using the marker and straight edge, beginning your measurement on the side of the ductboard with the stapling flap, 1 1/2 inches in from the edge of the board. Measure and mark the dimension of each of the sides of your ductwork on your ductboard, again starting from 1 1/2 inches in from the stapling flap.

Example: If your finished duct size will be 12" x 24", your cumulative width measurement will be 72" (12" + 24" + 12" + 24" = 72"). Your cumulative width mark will then be at 73 1/2" (72" + 1 1/2" = 73 1/2").

Your side measurement marks will be as follows, starting from the edge with the stapling flap:
A line at 13 1/2" (12" + 1 1/2")
A line at 37 1/2" (13 1/2" + 24")
A line at 49 1/2" (37 1/2" + 12")
This leaves the final 24-inch side between the third measurement line and the cumulative width line (73 1/2" - 49 1/2" = 24").

Make the cuts for the fiberglass duct sides. Use the female joint tool to cut the correct width of the duct on the cumulative width line. Use the shiplap tool to make the three remaining side cuts. For the first side cut, make a right-hand shiplap cut. For the second cut, make a left-hand shiplap cut. For the third cut, make a right-hand shiplap cut.

Prepare the stapling flap and fold the duct. Use the utility knife to carefully remove the insulation from a 1 1/2-inch width from the stapling flap edge. Be careful not to cut through the ductboard facing. Fold the duct work at the shiplap cuts and ensure the stapling edge sits properly on the rabbit cut.

Secure the closure joint. Use the staple gun and outward cinch staples and staple the stapling flap to the connecting side at 2-inch intervals. Cover the seam with pressure-sensitive tape. Rub the tape until the scrim pattern from the ductboard shows through the tape to ensure proper adhesion.