Remove existing ductwork where needed. Run the new flexible air duct under the house from the floor return to the underside of the air handler and attach to the metal vent box, or plenum (Image 1).
Note: If the air duct isn't long enough, it is possible to add extra length. Attach a second air duct to the first by peeling back the insulation and tucking it into the first section approximately 6"-10" and seal with duct tape. Fold the insulation back in place and roll on the outer layer (Image 2).
Use plastic ties, placed every few feet, to attach the entire length of the duct to the bottom of the floor joists; this keeps the duct up and away from the ground.
Seal the new flex duct to the new box with plastic ties, keeping the connection between the duct and the box as tight as possible. Once everything is in place, clip the end of the ties, seal everything tight with aluminum tape and burnish with a plastic rib.
To hold the new vent in place, it may be necessary to create a wooden frame or housing. Measure the opening and cut the 2x4s to size. Nail the frame into the opening and put the new vent in place (Image 1). Center, using 2" x 4" braces, and screw in place (Image 2).
Fill in the gaps between the floor and the new frame with insulated duct board. Cut the board to size, making sure it slides in and fits snugly on both sides of the frame (Image 1).
If needed, further seal the vent with mastic adhesive cement, which is painted on, ensuring no air escapes around the vent (Image 2).