Duct board consists of compressed fiberglass backed with a foil vapor barrier. The insulating qualities of compressed fiberglass reduces the temperature loss from air moving through the plenum and stops condensation from forming on the exterior surface of the duct plenum. Excessive condensation will damage finished materials surrounding the plenum and cause mold to grow in dampened wooden framing components. Properly cutting and seaming the duct board plenum will ensure minimal damage from condensation.
Set a piece of duct board on a firm surface with the foil vapor barrier sitting on the work surface.
Mark the duct board. Pull the tape measure along the piece of duct board. Mark 1-1/2 inches in from one end of the duct board with a soapstone. From the 1-1/2-inch mark, measure the height of your duct plus 1-3/4 inches and place another soapstone mark. Mark the remaining width and height locations, adding 1-3/4 inches to each dimension. Set the short side of the T-square against the side of the duct board. Align the long edge with a cut mark. Pull the soapstone along the edge of the T-square to mark the line. Repeat the process to mark each cut line, including the 1-1/2-inch mark placed near the end of the duct board.
Lay the duct board knife flat between the duct board and the foil vapor barrier with the tip of the knife pointing toward the 1-1/2-inch line. Slide the duct board knife along the width of the duct board. Drag the knife perpendicular along the 1-1/2-inch mark. Remove the 1- 1/2-inch piece of cut insulation.
Pull the V-groove tool along each marked cut line. Keep the flat section of the V-groove tool flat against the top of the duct board to ensure the notch is at the proper depth. Remove the notched insulation from each groove.
Fold the duct board to align the 1-1/2-inch foil flap with the far edge of the duct board.
Fold the foil flap over and staple the flap to the duct board. Seal the connection with a length of foil tape.
Cut two pieces of duct board 6 inches longer and wider than the inside opening of the plenum.
Place soapstone marks 3 inches in from each edge of the cut end pieces. Extend each mark with the T-square.
Remove the insulation from around the edges of the pieces with the duct board knife as described earlier.
Secure the end caps to the plenum. Push the cut pieces in each open end of the plenum. Slit the foil vapor barrier extending beyond the edges of the duct board with a duct board knife. Fold the ends of the foil onto the outside of the duct board. Staple the foil to the duct board. Tape the flaps down with foil tape to complete the duct board plenum.