It came with "joist spanners" that seem flimsy and possibly difficult to install. Alternatively it has mounting screw holes in the lip. I want to just cut out the old fan and expand the hole (I have been up in the attic and inspected. the existing joist/electrical etc before doing so). One side of the existing fan is already attached to a ceiling joist. I'd still use this point as a starting point and expand the drywall hole away from that existing joist accordingly to fit the new Panisonic box. My goal is to just cut the hole, install an new spanner joist to snug up to the other side of the box (would cross between the headers of the two existing walls) and just use some longer pan head wood screws to screw up through the "lip" with mounting holes right through the existing drywall ceiling and into the old existing joist and the newly install joist.
So in my example above after the new hole is cut into the drywall fit the larger fan, and new joist installed (parallel to the existing joist but for the other side of the fan hole). I'd push the new fan up through this hole until the mounting lip rested on the ceiling. I'd put screws in through this lip, up into the drywall and into the joist above. Then of course seal in the fan from above and replace the 3" exhaust duct with 4". Does that sound doable? I guess normally this lip would be between the ceiling drywall and joist but I can't see a downside to installing it directly onto the existing drywall.
What do you think?