Our two-storey house has a washroom on main floor, and it has no exhaust
fan. Don't want to make a fan on exterior wall (direct vent out), as our
winter is too cold.

I'm thinking to place a fan into interior wall, and get duct up to attic
inside drywall, then vent out from roof. I wonder if this is feasible. Are
there exhaust fans thin enough to fit into interior wall which is only 4-5"?
Is this too big a job to make through from main floor to attic (e.g. drill
holes on 2x4s)? If doable, I may pay to get an expert to do it.

Any input appreciated.

John