Knob and tube wiring explanation

Question

I have a house that is wired with knob and tube. I am looking to rewire everything but am very curious about how knob and tube works.
I took a video in my attic and wanted to see if anyone could explain to me what is going on here. I posted the video to YouTube, the link is down below, please check it out and let me know if there's any questions or more information you need and I will be happy to answer it !

https://youtu.be/CooIe9Ck62o

Answer 1

Knob and tube is quite simple. The wires are not insulated, or not significantly so, so the insulation is ceramic and air - tubes provide insulation when passing through framing, knobs hold the wires away from framing and air insulates the wires running between knobs. Sometimes boxes are built around the wire run if it's not inside walls, to prevent inadvertent contact.

It's rather similar to electric fences.

In most cases the original junctions are twisted and soldered. Rework later in the building's life may lead to all sorts of oddities.

Answer 2

The Knob & Tube is set up as individual wires. You have the hot and neutral wire running maybe 8 inches apart from each other. Each pair of wires is 1 circuit.

In some cases you may have a Multi-Wire Branch Circuit or MWBC, with 2 hots and a neutral (presumably the neutral would be between the hots) and 120V circuits would tap 1 hot and the neutral.

The first thing you should do is put those K&T circuits on AFCI breakers. These are Arc Fault detecting breakers, and an arc fault is that sparky thing that happens on a poor connection or accidental short. If you've ever hooked up speakers with the stereo's power on, you know the sound of an arc fault. Buy one AFCI breaker and then test each knob-n-tube circuit on that breaker. Sometimes the AFCI breaker does not work on an old circuit because it has a ground fault, or a crossed neutral or something weird. Most AFCI breakers also detect ground faults like a GFCI does. In a perfect world you'd find and fix that, but since you're about to tear it out, just avoid putting AFCI on that circuit.

You seem seem to have open joists. To give working, walking and possibly storage space, I would suggest decking over the attic with plywood strips ripped just narrow enough to get through the hatch. The wider the better. Make sure the area under the plywood is as full as it can be with insulation, but don't overstuff. The insulation is actually being done by the air, the job of the so-called "insulation" is to stop the air from circulating/convecting. That is why "cramming" does not work, as it doesn't increase the air.

Actually, there seems to be more loose insulation than could possibly fit between the joists. So I would screw 2x4's edgewise on top of the joists (pre-drill holes 1-1/2" deep the size of the screw head, then 3" Torx deck screws, so you go 1" into the joists). Then you have enough space to accommodate all that fill insulation under the decking.

You don't need 100% deck coverage, just enough to let you bury the insulation so you can see where you are stepping and so you don't "put your foot through" the ceiling and rain loose insulation and possibly an injured human onto the room below.

When you re-cable, every junction box needs to be in a location where it remains accessible. If it were me, I would put them in the quasi-floor and use steel boxes, since steel boxes and their blank covers are tough as nails.