I have a 1.5 storey house poured concrete foundation, main floor walls
built from a single layer of 'Angelstone' - actually hollow concrete
blocks with a stone look face, they are about 8*8*16 inches. Inside the
walls are strapped with 1" furring and some sort of fibreboard with a
coat of plaster. The house is 1950's and have beutiful handcrafted
coving so I don't want to destroy the interior cove or exterior 'stone'
look (even if it is fake it matches all the other houses).

I have already insulated the basement to r20 with solid foam and spray
foam around the rim joists. Also as part of major reconstruction the
attic has been insulated with spray foam on the old roof deck and a new
roof deck added with air gap before the final plywood and shingles (we
also spray foamed on top of the outside walls.

So the only part not insulated in the 8ft high main walls - I was
thinking of drilling holes in the drywall on main floor and spraying in
the foam in a can made for doors/windows so as not to expand the
drywall. Is it worth adding this 1" of insulation as the exterior wall
is currently estimated as only r4? I read some articles that said that
first inch gives a great reduction in energy and then falling return -
I would do it myself over a period of time so labour is free and as
only 1" gap not going to take much foam.

David