I know that code says no cellulose insulation with knob and tube wiring because of possible heat build up. I also know that evidence of it causing fires because of this is scarce and a few states allow cellulose if an electrician says it's in good shape.

As I air seal the attic before adding insulation there are about 10 knob and tube wires running above the kitchen ceiling, which is attic floor behind a kneewall. They were covered in blown fiberglass for probably the last 24 years.

I am having 2 different electricians come to give their recommendation and/or estimate of it it should be changed. However, since the wires come up an outside wall that's had spray foam for 8 years, go across the attic floor for about 10 feet and then disappear under a finished floor I don't see how replacing them is really that possible.

I'm going to see what the electricians say but I'm wondering about putting a piece of drywall a couple of inches above the wires, and boxing in the sides as well. That way it would be just like they are in an empty wall cavity except for the fact that there is insulation on one side and it's a ceiling cavity instead.



Nah. You're overthinking this, and the electricians are showing no evidence of thinking at all.

First, measure the wire: is it 14 gauge? 12 gauge? or something in between? Second, measure the breaker protecting the wire. Third, count the credible loads on the circuit. Fourth, use a handheld beeper to determine which wires are disconnected and thus irrelevant.

You only have a heat problem if there's enough current to heat the wire without tripping the breaker. If you're got 14 gauge wire, two space heaters and a 200 watt incandescent bulb and a 20 amp breaker -- problem. If you've got a circuit feeding a single light fixture with an LED bulb then no heat can possibly build up, because the wires will never get hot.

In general 14 gauge wire is good for up to 15 amps. 20 gauge wire is good for 20 amps. For safety margin, you can install the smallest breaker needed for your loads, even if it's smaller than the maximum the wire can support. Tables of wire thickness and current capacity are readily available online.

Then I'd inspect the K&T carefully, looking primarily for modern monkey splices. For a bonus, turn on all loads in the house and go up with a thermal camera, just on the off chance you've got an improperly made historic splice or modern wire nut getting warm. Satisfy yourself nothing is amiss.

Then put the insulation right back over the old wires. But take photos and leave a permanent marker above, warning any future attic explorer what lies below. You don't want anyone reaching through the insulation and grabbing a poorly insulated K&T wire.

Just remember: 14 gauge single phase knob and tube wire can carry just as much current, and will heat up just the same amount, as modern 14 gauge NM cable. Things get a bit more complicated with shared neutrals, but not much more complicated.

See also: Should old knob and tube wiring be replaced?

  • In "14 gauge wire is good for up to 15 amps. 20 gauge wire is good for 20 amps," did you mean "12 gauge wire"? – Joshua Taylor Nov 27 '18 at 0:43

Knob and Tube - cool, in a science museum way.

I think you have the right idea. Replacing them is of course ideal, but the money thing is a reality.

I would use concrete backer board, like in a shower, drywall can absorb water.

Glue the edges of the backer board together with some firestop caulk.

Build a 3/4 inch plywood frame around the outside this, of such a size you can attach the plywood to the backer board to the ply, firestop caulk or screws.

Any thing you can do to create a complete backer board box under the tube and wire setup is only better, use firestop caulk or foam.

Angle brace, simpson strong tie, do something so when the box is kicked by someone who can not see it under insulation and have it shift into the tube and wire setup.

Best of luck with whatever you do.

  • This sounds like significantly more cost and work than just replacing the knob and tube with modern wiring. Heck, even old aluminum or copper-clad aluminum wire is dangerous enough that I've been systematically replacing it throughout my 1926 house even when there was no "need" ... why take chances? – cathode Mar 17 '15 at 14:06
  • Cathode - love the name :) , in some places I use 'active_low'. I agree with you, about your assessment. What stopped from 'romex' as being my answer was the comment about the knob and tube going under the finished floor, with no easy solution. As well as 10 wires being involved, that is a bunch of circuits to trace. This job really seems to scream 'Romex and Wiremold'. Thanks for the nudge toward sanity on this question. – Some Guy Mar 17 '15 at 14:38
  • Thanks :) The old knob and tube wiring could potentially be used to pull new wire through, although it might be difficult. Surface mounted wiremold ducting is definitely a good alternative though. – cathode Mar 17 '15 at 16:05
  • Thank you for the responses. I have 2 electricians coming in the next couple days to give opinions and estimates. As I see it the options are: 1. Box it in as I described, 2 replace the breakers with AFCI breakers, 3. run new from the panel up the wall and across the floor, with junction boxes above insulation level just before they enter the floor. – user20127 Mar 18 '15 at 12:00
  • Please let us know how they solve the problem, and cost if you would. I live in annapolis, maryland. I have done some work on things that seem almost as old as the country itself. - Also, when the wire comes out, might want to save a knob and a tube, just to show others what they look like from 'back in the day:) – Some Guy Mar 18 '15 at 12:19

OP here, I had 2 electricians come look at the situation.

1st: kept repeating how it's against code, how the walls are spray foamed, oh my, you would have thought a kid was there with his hand in an open junction or something, the guy was just beside himself with it. He suggested replacing the wires from the panel, up the wall and across that floor. He went to his truck to call the owner and came back, saying they didn't want the job or liability if I didn't want all the wiring in all the walls replaced.

2nd: this evening a different electrician came, he first suggested running new wire across the space but said, really, it's not a big issue. I told him my three ideas: new wire from panel to there as suggested by #1, boxing it in, or arc fault breakers. He suggested boxing it in with plywood, thought that would be good, said some inspectors have been ok with it. Said adding arc fault breakers would be pricy trying to match neutrals with the other wires and would not be a gurantee. I asked for a rough estimates for arc faults, he said with all the time tracing it, plus install he thought $1,000 to $1,500. He was quite talkative and said if it were him he'd just box them in.

I'm pretty set that that's what I'm going to do.

  • Your first electrician did what I would do, not take the job because spray foam in the walls is a major code violation with K&T, but building a box around the boxes to keep the insulation separated needs to be done on top of needing separation it is not just for heat build up but if a splice fails and throws hot metal it reduces the chance of a fire. The boxes also provide a location for the wire runs. – Ed Beal Jul 24 '18 at 16:19

Code doesn't say that anymore, at least not in WA and OR.

Serious study was done on the question of whether blown insulation on K&T was really having an impact on house fires. The studies determined it was not. And so states have been rescinding their laws against blown insulation with K&T.

Of course AFCI is a magic bullet that intercepts many wiring faults, and I would put it on any K&T.

Sometimes K&T has problems with neutrals being crossed among several hots -- that was Wrong then, just as it is wrong now. However MWBC is a method where two hots share 1 neutral, and that was legal then, and it's legal now. The hots simply must be put on opposite poles, and the breakrs handle tied. Regardless, the newest AFCIs don't care about shared neutrals and tangled up neutrals are no longer a reason not to install AFCI.

If the house has blown insulation, you might downbreaker one size, merely out of an abundance of caution.

Other than that, I would continue to use it indefinitely.

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