The house I recently bought did not originally have a bathroom fan,
however one of the previous owners added one. They did a half-assed
job like every other project they did to the house. The house is a
standard ranch from the late 70's with a very low slope roof.

First, the fan box is not attached to the ceiling joists. It simply
sits on top of the drywall. However, one edge of the ceiling hole is
right up against a ceiling joist. That's good. Second, it's obvious
the fan is not pulling air like it should. There is a hose attached
to the box, but I'm not sure where it goes. When the fan is run, it
makes a vibrating noise and there is no pull whatsoever on the
bathroom door or any other sign it's actually sucking air. If I take
the fan out and plug it into an outlet, it runs smoothly. I would
like to install a roof vent, and attach the box like it should be.

There is little over 1 foot of space between the ceiling drywall and
the roof, as seen through the ceiling hole. In addition, there is
lots of blown fiberglass insulation in the attic. Needless to say, I
hate thinking about going into this attic. I could easily install the
vent by not going into the attic, but attaching the box is another
story.

My question is: is there a fan box available that can be mounted from
inside the bathroom? I'm thinking of something like the "old work"
electrical boxes. I have thought about simply running screws through
the box into to joist. If I must go into the attic, I can probably do
that provided I wear a respirator and some sort of disposable
coveralls because of the fiberglass.

Thanks for any ideas/insight.