It would help us if you could give us a little more information. For example:
* Is your home made of stone, stucco over stone brick, wood framed with siding, etc.?
* If stone or brick, do you have interior walls framed with wood?
* Is your attic finished?
* What is the structure of your roof, and what kind of shingles do you have?
* What area do you live in?
The more details you give, the better chance someone will have a good recommendation for you.
I don't really want to remove the plaster and lathe and am looking for a solution for filling the walls. One thing I'm not sure is wheather or not there are fire breaks bridging the wall studs. Probably not, but here's what I've considered so far.
1) Remove several strips of siding and drill holes to blow in wool from the outside. I would need a professional for this job. To do it from the inside would be a mess.
2) Drilling interior holes at the wall top and filling with PFC. (small styrofoam balls) The top of the wall is behind a drop ceiling so I could get to the holes and refill after a year when the material has settled.
3) Drilling several holes and injecting expanding foam. I saw the tail end of a Bob Vila show where they where either injecting or spraying some kind of expanding foam into walls and floor joists. What were they using?
I don't want to spend a lot of money and am looking for a solution that I can execute myself. Any ideas on which way to go, or have any better ideas?