How to Run HVAC Ducts in Floor Cavities

Remove all furniture and other items from the room. Seal the doorways with plastic sheeting and duct tape to minimize dust drifting into other rooms. Cover the floor with plastic sheeting or canvas drop cloth.

Use a stud finder to locate the floor joists in the ceiling. Mark the locations with a pencil.

Use a utility knife or chisel to cut away the drywall between the floor joists. For best results, cut the drywall at the center of each wooden floor joist. This allows a firm, wooden support on which to screw in the replacement drywall sections. Remove the drywall material slowly and carefully, watching for electrical wiring or pests' nests.

Measure the length of ductwork needed to fit within the floor cavity. In most cases, you need to connect at least one section of rectangular ducting with another section.

Connect two ducts together by abutting the hooked end of one duct across the plain end of the other duct. Insert a drive cleat over the two hooked ends to secure the ducts together at the seam. Bend the remaining ends of the drive cleats around the duct, forming a "U" shape that encapsulates each side of the seam.

Paste duct mastic over the seams and allow to dry. With the aid of an assistant or two, raise the ductwork into the open floor cavity. Loop strips of webbing straps -- polypropylene straps -- or metal straps under the ductwork. Nail or screw the straps to the wooden floor joists. Be careful to tuck the ductwork fully within the floor cavity or you will be unable to repair the ceiling.

Repair the ceiling by measuring and cutting new sections of drywall to match the portions you had removed. Put the ends of the drywall over the wooden floor joists and screw them into the wood.

Cover the drywall seams with mesh tape and apply several coats of joint compound. Sand the seams smooth. Repaint the ceiling.