How to Fabricate a Fiberglass Duct


Fiberglass duct comes from the distributor as a 120-by-48 inch board. The duct board also comes with the option of 1-, 1½- and 2-inch thickness. Installers fabricate the duct on site with specially designed color-coded knives. The thickness of the fiberglass board determines the required set of knives. In many homes leaking ducts account for 10 percent to 30 percent of a home's cooling and heating costs.

Things You'll Need

  • Tape measure
  • Duct square
  • Staple flap duct knife
  • V-groove corner-cut duct knife
  • Duct knife
  • Metal-backed duct tape
  • Staple gun
  • Staples
  • Duct sealing mesh
  • Duct sealing mastic
  • Determine the fiberglass duct's inside dimensions. Measure the unit tie-in size and the space allowed for duct installation with a tape measure. Take the outside dimensions and subtract twice the thickness of the fiberglass duct board.

  • Lay a sheet of duct board on a clean, solid surface with the outer liner down.

  • Set the duct square on the duct board. A duct square resembles a 4-foot-tall carpenter's square with the measurement marks across a 3-foot long bottom. The 4-foot section acts as a straightedge for the duct knives to follow.

  • Add one half of the duct board's thickness to the inside dimension width. Line up the corresponding duct square measurement with the edge of the duct board sheet.

  • Place a V-groove corner-cut duct knife (red) on the duct board next to the duct square. A V-groove corner-cut duct knife has two blades that come together in a point.

  • Apply firm pressure to the corner-cut knife. Push it across the duct board using the duct square as a guide. After completing the cut, remove the triangle-shaped scrap pieces.

  • Slide the duct square along the bottom of the duct board until the duct square's measurement marks line up with the inside height measurement. Cut the duct board with the corner-cut knife and remove the scrap.

  • Slide the duct square until the measurements on the duct square line up with the width measurement. Following the duct square, cut the duct board with the corner-cut knife. Remove the scrap.

  • Slide the duct square until the measurements line up with the height measurement plus one half the thickness.

  • Place a staple flap duct knife (blue) on the duct board. A staple flap knife has three blades, one shaped like an L that extends halfway through the thickness of the board and two straight blades that extend to the duct board backing paper. Cut the duct board with the staple flap knife.

  • Cut off the excess duct board with a duct knife. Use the far right groove as a cutting guide. Remove the two pieces of scrap from the duct.

  • Fold the duct board, keeping the flap on top. Mate the two ends. The extra one half thickness on the first cut will fit tightly into the second cut made by the staple flap knife.

  • Tack the flap in place with metal-backed duct tape. Three 4-inch sections of tape, placed near the ends and in the middle, will temporarily hold the duct together.

  • Attach the flap with a staple gun. Place a staple every 2 inches.

  • Cover the flap seam with metal-backed duct tape. Use the knife's handle to press the tape into the duct.

  • Cover the duct tape with duct sealing mesh. Cut the mesh to size with a knife.

  • Seal the duct with duct-sealing mastic. Use a brush to spread the mastic over the mesh and any rips in the outer layer.

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  • Photo Credit air conditioner, conditioning image by Greg Pickens from
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