I have some new AC ducts in our new house that will soon be concealed behind new drywall, but before the drywall goes up, I wanted to run some conduit from the upper floor down to the basement for cable and network. Unfortunately, in the space between the two ducts (where I pretty much would have to put the cables, there is, attached to the floor joists a run of knob and tube wiring. I tested it with a voltage tester and it doesn't appear to be hot, so I'm assuming it's the neutral line (I can see where it's partner live line is, however). It looks to be in pretty good condition with the insulation in tact, but I'm leery about trying to run flexible aluminium condition right next to it. I have limited space to work in and limit options on securing it, but obviously I don't want the potential for the conduit to end up live!

How worried should I be about this? If I secure the conduit away from the k&t, would I be okay? Or should I just forget the conduit altogether and just drop (insulated) cables down the hole and hope I don't need to change them or add more at a later date?

  • 1
    Why not PVC? Will allow you to pull new runs easier in the future.
    – DMoore
    Aug 20, 2014 at 15:06
  • @DMoore: Because, regretfully, that's what I bought. I may well go back and get plastic instead. Aug 20, 2014 at 15:16
  • At SOME point you really should consider replacing that k&t with NM-B. You know that, right? This might be a good time? Aug 20, 2014 at 15:39
  • 1
    Knob and Tube wiring is a serious hazard to live safety. Knob and tube wiring is not just an inconvenience like an old drafty window. It really needs doing. The AC won't keep you cool in the event if there's a fire, and if there is a fire, you and/or your loved ones may wind up dead.
    – user23752
    Aug 20, 2014 at 16:21
  • 1
    @benrudgers the Mike Holt forum is for professionals, not DIYers. They likely will not be very helpful.
    – Tester101
    Aug 20, 2014 at 23:03

2 Answers 2


You could use "smurf tube" (Electrical Nonmetallic Tubing) or rigid PVC to have a non-conductive conduit. Be sure to bond any metallic conduit that is separated by such non-metallic conduit, or just use nonmetallic the whole way with ground wires. I suspect you are not supposed to contact the K&T in any case and should be securing the wires or conduit to prevent such contact.


I make the assumption that knob & tube insulation will fail. Maybe it will, maybe it won't, why take the chance. With stock K&T insulation failure is no problem: the ceramic tubes will outlast any plastic insulation anyway.

But adding a metal pipe in the same cavity? Why? Use plastic conduit.

See also https://diy.stackexchange.com/a/20279/5960

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