My wife and I recently bought an older home that we would like to renovate and turn into a rental. The house was built in ~1938; it is a nice, all brick 2-story with wood framing, so it has wonderful bones. Due to its age, I fully expected to find plaster walls. However, when we cut into the walls to begin adding boxes for electrical upgrades and smoke detectors we discovered something odd (see attached photos). The wood framing was first covered with what appears to be 3/8" drywall, then 1/2" sheet rock, and then the final coat of plaster. I don't have alot of experiece dealing with homes of this age, but I thought it was odd to find a layer of drywall. I assume this wouldn't have been typical construction for the time period, right? We also intended to insulate the walls with dense pack, blown in cellulose insulation, but now I'm concerned about how difficult it's going to be to cut through the rock lath in multiple stud locations every 24" OC. The rock lath is very difficult to penetrate and I don't know how many carbide hole saw bits it will take to do what I need to do for insulation access. Then, with this extra layer of drywall beneath, I'm not sure how I'll patch all the holes up in any type of efficient manner. If we were only doing a few penetrations, I wouldn't be too worried, but we're looking at upwards of a 100 holes (2 per stud cavity, per floor). Fortunately, the house has a walk up attic with a tongue-and-groove flooring that is open on both ends of the floor joist spaces, so I can easily get insulation in those cavities (even if I have to drill some intermediate holes in the floor boards.
Should we bag the idea of insulating the walls, and just insulate the attic since that's where the majority fo the heat loss is any how? BTW, the house is heated w/ a cast iron sectional oil-fired boiler feeding how water to radiators. Any thoughts would be appreciated!