One of the things you really have to be prepared for when buying an old house is that you’re gonna find things behind the walls that aren’t ideal. Sometimes it can be as simple as some subpar framing or failing insulation, but you’ll also regularly find larger problems like knob & tube wiring.
Knob & tube is an old wiring method used back in the early to mid-1900s. It’s essentially cloth wrapped wiring that is run between a series of plastic knobs mounted on the framing. When the wire needed to go through a 2x4, they would drill a hole and cover the wire in that section with a tube, hence the name.
But the problem with this method is the wiring is extremely unprotected. Unlike Romex wiring, which is the current standard, knob & tube wires are easily damaged. And there is no metal conduit to protect them. So if a rodent decides to chew on one of them, it won’t be pretty (for the rodent or your house).
And for those in the “out of sight, out of mind” camp, I have some bad news: if your insurance company gets wind of the substandard wiring (and they have ways of finding out, believe me), they will simply drop you from your policy.
So if your home inspector finds active knob and tube wiring on the house you want to buy, please don’t ignore it. You will likely need a total rewire of the home in the near future. And that can range anywhere from $15 - $50k+, depending on the size of the house. Ouch!