Cable fill and ceiling fan wiring

Let me apologize in advance for the long setup!

I'm installing a ceiling fan in a bedroom where there has been no existing ceiling fixture. I want it to be controlled by a wall switch that currently switches a wall outlet, and I would like that outlet to be unswitched. I had planned to put the fan and light on the single switch rather than run 3-wire cable because I didn't want to bother switching out the switch box for a 2-gang. Above the ceiling is an unfinished attic accessible by a scuttle. Up there, at the location of the switch, is a hole in the top plate with two cables going into it (12-2, aluminum by the way). I wrongly assumed one was heading off to the outlet.

When I opened up the switch, I found the two cables entering the top and a third entering the bottom of the box, carrying power away to the outlet. This switch box, apparently, is also being used as a junction for a cable heading off to parts unknown. It's also pretty jam-packed, and by my calculations does not have room for another cable. It already has 6 12 gauge wires; 2 more plus ground plus switch by my math =24.75 cu in required; I don't think 1-gang boxes even come that big. Plus, on the practical side, I'll be using those purple rectangular mini-bus bars for connecting copper to aluminum, and those take up even a bit more space than wirenuts.

I think the only solution I can see is to go ahead and put in a 2-gang box, 3-wire cable, and switches for both fan speed and light. By my math that would add up to 31.5 cu in, and I can get a 34 cu in old work box. Before I go ahead with that plan, is there any other solution I'm overlooking? I had briefly toyed with the idea of putting a junction box in the attic for the two wires that go into the wall, and running a jumper section down from them to the switch... but the cables have no slack and are stapled to the studs in the wall (as far as I can tell by tugging a bit). I really would prefer not to get into anything that involves replacing drywall and painting.

By the way, my wife is a bit antsy (and so am I, honestly) about the idea of old-work boxes supported by drywall alone. Is there really no box that you can attach to the stud, like by screwing through the wall of the box? I guess nobody wants an ungrounded metallic screwhead inside the box. Still... gee.

 

It's not that hard to remove the stables that hold the cable, to get the slack that you need, as long as you aren't, in a hurry. There were times I had to do it, to install a ceiling fan. Look for existing junction boxes that can be used either as a feed or just another place to transfer juice. Are the existing cables BX or Romex?

 

Misguided worry. Old work boxes hold well. Your only handicapping yourself by not using old work boxes. The first thing I'd do on a job like this is remove the existing box so pulling the new cable is easy. If you just have to have something with screws use a Smart Box old work box.

Good thinking to reject that idea. No sense looking for trouble.

A two gang box is good but a deep single gang might work also.

 

Thanks everybody! I ended up putting in the two switches after all, and am glad to have separate switches for light and fan. The exciting part was wedging the new box in between the stud and the nearby door framing member...