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I have a room that has some really ugly cheap panelling on the walls. I want to rip it out & replace with drywall. While I'm at it, I'd like to add soundproofing insulation between rooms & thermal insulation on the outside walls.

However, the house has knob & tube wiring. Is it safe to leave the wiring alone for this project? (Obviously if something must be done, doing it when the wall is open is best.)

Niall C.
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tomwhipple
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2 Answers2

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The answer changes depending on location.

Because of a complete lack of actual fire incidents, four Western USA States: CA, OR, WA and ID petitioned for an exception to NEC 394.12 and permit insulating over Knob & Tube Wiring. In some places you must first file a Knob-and-Tube Wiring Safety Report, and everywhere else it's a good idea.

It is a complete myth that Knob & Tube must be air cooled. Instead measure the gauge of the wire and put in an appropriate AFCI breaker for the gauge, and you're done. 100 year old copper is just as good as modern copper, in regards to household use. You should remove from the K&T any high load devices. What's left, especially in the era of CFL's and LED's, won't draw enough current to cause a problem.

The insulation of the era was not all that good, and perhaps for that reason K&T does not depend on it. Ceramic tubes will outlast civilizations, and hold the wires apart even if the insulation is damaged.

You can even add an extra margin of safety by underrating the K&T (the opposite of depending on air cooling). For example if you find 12 gauge wire, put a 15 amp AFCI in place, and you're more than done. For 14 gauge wire, a 10 amp breaker might do nicely, even if they look at you funny at Home Depot. Home Depot probably won't have the right breaker, but you can brave DIY scorn at Grainger, or just order online.

See

You should, however, replace any fuses. Those are dangerous, even with Type S fuse bases.

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Bryce
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    That said, while walls are open, it's so cheap and easy to replace the wiring, that you should do it. – Bryce Mar 28 '14 at 21:57
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    The only downside is that handling K&T often leads to further deterioration of the insulation on the parts that are left. – bib Mar 29 '14 at 02:17
  • Any K&T run inside those walls should be cut back to a K&T junction. A new circuit should come from the breaker box to serve those loads. K&T deterioration is no big deal, except at metal junction boxes. – Bryce Mar 29 '14 at 02:19
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    That K&T is going into a box at some point and the wires will not be separated anymore. I wont touch K@b the few times I just tried just to replace an outlet or switch the insulation started to come off. – Justin K Mar 29 '14 at 03:46
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    @JustinK I recommend running new circuits back to the breaker box: don't splice to old K&T, even in a box. Just chop bits of the K&T hydra off, making it smaller and smaller over time. – Bryce Mar 29 '14 at 06:52
  • You really love knob and tube. – Tester101 Mar 29 '14 at 23:11
  • @Tester101 true: I do prefer soldered connections over wire nuts, and a system that's robust against wayward screws and nails. – Bryce Mar 30 '14 at 06:03
  • @Bryce, was that sarcasm? Every time I've touched old K&T wiring the insulation on the wires literally crumbles and falls aparat, not exactly confidence-inspiring, especially in light of all the questions about covering the wires with insulation. – Craig Tullis Nov 29 '14 at 04:10
  • Also, hard not to notice on that Knob & Tube Wiring Safety report how it requires an inspection prior to blowing in insulation, and that the attic has to have a warning posted about the extra caution required moving around in the attic due to the presence of knob & tube. Romex or MC seems a lot less fragile, to me, with all due respect. ;-) – Craig Tullis Nov 29 '14 at 04:12
  • No sarcasm. K&T does not even need the insulation to perform, where NM cable depends on it. The K&T I happily leave in place is generally inside walls and closed ceilings where it will cause no harm, insulation or not. For where it goes into a junction box I slip new loam or shrink tube over it, and problem is solved. – Bryce Nov 30 '14 at 09:26
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If you can insulate the walls without a "material that envelops the the conductors", then sure. But I would think that wouldn't be a very well insulated wall. Knob and Tube wiring is meant to have an area of free air about it. Insulation encroaching on this space could cause the wiring to overheat.

National Electrical Code 2014

Chapter 3 Wiring Methods and Materials

Article 394 Concealed Knob-and-Tube Wiring

II. Installation

394.12 Uses Not Permitted. Concealed knob-and-tube wiring shall not be used in the following:

(5) Hollow spaces of walls, ceilings, and attics where such spaces are insulated by loose, rolled, or foamed-inplace insulating material that envelops the conductors

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Tester101
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  • Except in four western USA States: CA, OR, WA and ID, where insulation is legal after inspection. Remember that K&T with the proper fuse is equal to or better to modern wire in terms of heat dissipation. See http://diy.stackexchange.com/a/40489/5960 . – Bryce Nov 30 '14 at 09:28