If your house went up before about 1950, there's a decent chance there's still knob-and-tube somewhere in it. On the Northside, around the University District and in the lower Rattlesnake we run into it constantly. People hear the phrase and assume the house is a firetrap. That's usually not true, and the real answer is more specific than that.
What it is
Knob-and-tube is the original residential wiring method: a hot wire and a neutral running separately, held off the framing by porcelain knobs and passing through joists in porcelain tubes. The wire is copper, often with a cloth or rubber jacket.
The design is not stupid. Running the conductors apart with air around them was how it shed heat. Installed well and left alone, plenty of it is still doing fine a century later.
When it's genuinely a problem
Four situations. These are the ones that matter:
- It's buried in insulation. This is the big one. The system relies on open air to cool. When somebody blew cellulose into an attic in 1978 and covered it, you lost the cooling. This is a real fire risk and it is extremely common here, because everybody insulated their attic at some point.
- Somebody spliced into it. Original installs were careful. The additions made in the decades since, often not. Splices in junction-box-free midair with electrical tape are what we find.
- The insulation on the wire is failing. Rubber gets brittle and crumbles, especially near heat. If the jacket flakes off when touched, it's done.
- It's carrying modern load. It was never meant for a window AC unit, a 1,500-watt space heater or a dryer. And it's ungrounded, so anything needing a ground shouldn't be on it.
When it's fine for now
Original, untouched, not buried in insulation, jacket intact, and running lights and the odd lamp. That is not an emergency. It is something to plan around, not panic about.
The insurance side
This is often what actually forces the issue. A growing number of carriers won't write a new policy on a house with active knob-and-tube, or they'll require an electrician's letter about what's energized and what isn't. If you're buying or shopping policies, ask before you're surprised at closing.
You don't have to do it all at once
A whole-house rewire is a big expensive job and most people can't swing it in one shot. You can stage it:
- Start with anything buried in insulation, since that's the actual hazard
- Then the circuits carrying real load, usually kitchen and anywhere a heater gets plugged in
- Add grounded circuits where you need grounded outlets, instead of rewiring the whole room
- Leave intact, lightly loaded lighting circuits for later if the budget says so
What we won't do is tell you the house needs a $20,000 rewire on Tuesday when what it actually needs is two circuits pulled out of the attic insulation. If you want somebody to look and tell you straight, call (406) 313-6180.