How can I improve the insulation of lath and plaster clad exterior walls?

I'm having the two living rooms in my victorian semi knocked into one and want to improve the insulation at the same time as the front room in particular gets really cold (it has a bay window which probably doesn't help). The outside walls are solid brick with an air gap and then lath and plaster mounted on battens (I think). I'm not worried about ripping off all the lath and plaster but I want something that's going to have the greatest effect with the minimum cash and effort.

It can be blown in from the interior through holes made at the top and bottom . They can be smaller than 3" inches. There is a special nozzle for this type of application. You can do this yourself - I did...

As the battens on your wall system is probably @ 3-2" , sometimes less on a Victorian. Blown insulation is probably going to be more costly in the long run based against energy costs in the both the present and future. The dollars you spend now are more than worth the effort.

You can get a quote from an insulation contractor who will cut about a 3 inch hole in the top and bottom of each stud cavity and then fill it with blown-in insulation.

As heat rises, most of it will escape through the attic. Typically I am more concerned with cold air penetration through windows, and heat loss through the roof. Try caulking, windows, or replacing them, and insulate the attic better, you might be surprised at the results. But to answer your question, the first 2 are correct