Attic insulation needs to be moved out of the way to cut a ceiling joist.
Attic fans provide an energy-saving answer for homes in areas with low humidity, high daytime temperatures, and rapid cooling at night. Instead of running an air-conditioner in the evening and overnight, you can turn on the attic fan to bring the cooler air indoors -- at one-fifth the cost. Many models of attic fans are designed either to fit within existing joists 16 or 24 inches on center, or to perch atop an uncut center joist. Other models require you cut out a joist and to construct a simple box to hold the attic fan.
Identify a logical location that places the fan in the center of the house. Mark the area of ceiling drywall to accept the fan louvers, centering it on a joist determined with an electronic stud finder. Mark the louver corners based on the template provided on the attic fan’s shipping box. Drill a hole with a ¼-inch drill bit at each corner.
Check the area in the attic demarcated by the drill holes to make sure it is free from electric cables and pipes. Look for sufficient clearance above the ceiling to accommodate the depth of the frame for the fan blades. Roll back insulation batts to reveal the ceiling joists. If the insulation is loose fill, consult with the manufacturer on whether an attic fan is advisable.
Return to the floor below the area and mark the louver area with a pencil and a straightedge. Score the lines with a utility knife. Saw the drywall piece out with a drywall saw and set it aside.
Return to the attic. Measure, mark and cut with a reciprocating saw the joist, cutting an extra 1 ½ inches back from the drywall edges to account for the headers you will create. Measure the distance between the existing uncut joists and cut two pieces of 2-inch lumber, matching the dimensions of the existing ceiling joists, to fit snugly and act as headers. Bang the headers into position. Check that the headers are vertical, using a 12-inch spirit level.
Apply clamps so the headers line up perfectly with the edges of joists. Nail 12d nails through the joists and into the ends of the headers.
Clamp the ends of the cut joist to the headers. Nail 12d nails through the headers into the ends of the cut joist. After you’ve cut and boxed the ceiling joists, proceed to make a platform or plenum and install the attic fan per manufacturer’s instructions.
Parrish, Rogue. "How to Cut Ceiling Joists for Attic Fans." Home Guides | SF Gate, http://homeguides.sfgate.com/cut-ceiling-joists-attic-fans-25438.html. Accessed 20 April 2019.