If you are upgrading the insulation from the inside you'll have whatever is the inner wall out anyway, it's pointless otherwise; if you're doing it from the outside (possible with some roofs) it doesn't matter; and filling the wall cavity is done from the outside too, unless you have a really weird house no wiring goes there.

> If you are upgrading the insulation from the inside you'll have whatever is the inner wall out anyway

In Philadelphia in 1996, I had a company blow in insulation to my wood-framed, plaster+wood-lathe covered cavity walls without removing the inner walls. They created a series of 4"/10cm holes at the top of each bay and blew the insulation in that way.

ps. I later found out that this is a terrible idea for vapor control reasons.

Yeah, that's basically why just putting up new walls is the long term solution. And as a bonus you can put in some sweet CAT6 cabling and enough electrical sockets while you're at it. :)

Another benefit: when you take out the inner walls in an old house, you are going to find layers and layers of… stuff. Like 2cm of styrofoam (insulation in a nineties way) and a new layer of drywall on top of whatever chipboard/grout/newspapers where put in in a distant past (at least in my old house).

Massive project though.

You don't have to remove all of the lathe and plaster. Blown in insulation after upgrading the electrical. Fish poles and long cuts repaired afterwards.