I have an 1890's queen anne that has a lathe and plaster system between the wall studs and rafters. This leaves about 2.5 inches to the interior face of the wall surface.

We have removed the interior plaster walls to expose this internal plaster system, but before we rip this out and re-insulate, we want to see if we can keep this and add to it with new insulation such as spray foam or cellulose.

This system seems to give good noise and insulation value as our heating bills are OK.

I am looking for R value advice on this system?

I am also looking for advice as to the best insulation to add-on to this.