Running a metal cold air return duct between studs requires knowledge of both HVAC systems and the components involved in framing. First, building codes in many areas require that the cold air return line gets external insulation, and therefore you will need to leave a gap for the insulation around the duct return line. After you install the duct through the studs, you will need to add framing around the top and bottom of the ductwork to allow for a wall finish such as drywall.
Measure the centerline of the ductwork from the nearest parallel wall with a tape measure.
Place a mark on the floor in front of the wall that you need to run the ductwork through, measuring from the same wall that you used for the centerline of the duct.
Measure out from the centerline half of the duct width plus 1 inch and place a pencil mark on the floor. Repeat the process for the other side of the duct. For example, measure 7 inches from both sides of the centerline for a 12-inch wide duct.
Compare the marks on the floor with the wall studs. Mark the wall stud that you need to remove from the path of the ductwork.
Measure the bottom height of the ductwork.
Transfer the height of the ductwork to the stud that you marked in Step 4.
Measure up from the mark you placed on the stud in Step 6 and place another pencil mark at the height of the duct. From each mark place another mark 2 3/4 of an inch up from the top mark and down from the bottom mark.
Extend the lowest and highest mark around the stud with the carpenter's square and pencil. Transfer both marks to the studs located on either side of the stud you need to cut.
Cut the stud on both extended lines with the reciprocating saw.
Measure the distance between the two studs on either side of the stud you cut in Step 9.
Mark two pieces of framing lumber to match the dimension you measured in Step 10.
Cut the framing lumber to length with the circular saw to make two heads for the framed wall.
Align the bottom of the lower header with the low line you transferred to the studs in Step 8.
Secure the bottom header to the studs with four 8-penny nails, two nailed through the vertical studs into each end of the header. Repeat the process for the top header, aligning the top edge of the header with the high pencil mark.
Measure over from one of the two vertical studs attached to the header 14 3/4 of an inch and place a pencil mark on both the top and bottom headers.
Align the lower portion of the cut stud with the pencil mark on the bottom header and drive two 8-penny nails through the header and into the stud to secure the cut stud to the header. Repeat the process with the top portion of the cut stud and the top header.
Run the ductwork through the opening you created in-between the studs.
Insulate the portion of ductwork that runs through the studs. Seal the insulation seam with metal foil tape to secure it to the ductwork.
Measure the height between the top and bottom headers.
Cut two pieces of lumber to fit between the headers, known as jack studs.
Place the jack studs on both sides of the insulated ductwork, leaving a small gap between the insulation and the jack studs.
Secure the jack studs by hammering two 8-penny nails through the heads and into each end of the jack studs.
Repeat the process for each wall that you encounter while running the ductwork.