Knob & Tube Under Insulation

A revised graphic explaining the reason for the NEC requirement prohibiting the covering of K&T with insulation.

NEC 394.12 Blown-in, foamed-in, or rolled insulation prevents the dissipation of heat into the free air space, resulting in higher conductor temperature, which could cause insulation breakdown and possible ignition of the insulation.


Hold on, hold on.
There’s a problem here.
Where is the fact check committee on these illustrations and concepts?

I live in a state with a procedure for insulating over knob and tube wiring, and the issues have absolutely zero to do with heat. The NEC is in force here, as ammended.

The heat concern has to do with the ampacity of the wires, the so-called “free air ampacity”. If the circuit breaker or fuse operating the line segment matches the gauge of the wire, there is no problem with insulation and heat. None. No problem. As in, zero problem. In fact modern wire as a bundle of three conductors has more inherent vulnerability, compared to K&T.

To be fair here you’d have to say the same thing about insulating over modern wire: make sure it’s not getting hot before you insulate. Yet of course we already do that, as an inspector will call out (if seen) 40 or 50 amp breakers on 14 (or less) gauge wire, or other mismatches between wire and OCPD.

Knob and Tube Form.RECO.pdf (252.0 KB)

The far bigger issue with insulating over K&T is it could hide flaws in the wire’s sheathing, particularly if that wire can now sag and contact a plumbing pipe. For that reason ensuring bonding of the hot, cold and gas lines at every water heater provides a last resort line of defense. There can be no dielectric preventing a ground path on any pipe. One could extend this concept to mechanical systems also, such as exhaust vents.

We insulate over knob & tube all the time. Every day. Around here.

Not all cities or counties adopted the NEC in their codes. It’s up to the individual inspector to use the graphic if they feel it’s appropriate.

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No surprise for a state that is the bane of society!!

Morning, Bryce.
Hope this post finds you well and in good spirits today.

Bryce, look at the big picture.
A: InterNACHI does not have to vet illustrations provided by members.
B: K&T hazards. 1. Unsafe modifications are far more common with K&T wiring than they are with Romex and other modern wiring systems. …
2. The insulation that envelopes the wiring is a fire hazard.
3. It tends to stretch and sag over time.
4. It lacks a grounding conductor.

Hope that helps.

There is nothing to fact check. Randy created a graphic based on the NEC {394.12(5)} which has does not allow insulation around concealed knob and tube wiring. The fact that this is permitted by local amendment where you are doesn’t make the graphic incorrect.

I took a look at the link in your post, are there actually licensed electrical contractors that sign off on K&T buried in insulation and say that it’s safe?



And it’s not a very local amendment, it’s the entire West Coast.
This was done after a review of fire history showed no problem with K&T insulation.

Illustrating is fine, but illustrations are only helpful in a context.

Inspecting K&T largely consists of looking for handy-andy’s bizzare interconnections. The tubes and knobs will outlast by a factor of thousands the plastic in modern wire. The conductors themselves typically have pretty crappy insulation (depending on the era), but it’s not part of the system. The rules seem mostly designed to protect future workers from encountering a bare conductor between tubes.

Again there is ZERO problem with heat. Heat is a red herring in this discussion and in the illustration.

I’m trying to figure out how they got that new lumber wrapped around 100+ years old wiring :slight_smile:

I assume this was copied directly from the California building code?

"The wiring shall be surveyed by and electrical contractor…
“All accessible areas in the building where insulation has been installed around knob and tube and shall be posted…”

No offense, but this doesn’t read right. Had some parts been omitted from the document you copied?

Where’s the direct link to this Copy&Paste that was edited by someone? You?
It full of grammatical errors and bad parsing!! No way the states attorneys allowed this to fly through!!

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Why the insinuation?
The link to the original government document is above, the clip is unaltered.

The State of Washington apparently had better proofreading when they adopted a similar position:

WAC 296-46B-394 Wiring methods and material
Concealed knob-and-tube wiring.
001 Knob-and-tube wiring.
Article 394 NEC does not prohibit the installation of loose or
rolled thermal insulating material in spaces containing existing knob-and-tube wiring provided that all the following conditions are met:
(1) The wiring must be surveyed by an appropriately licensed
electrical contractor who must certify in writing to the department
that the wiring is in good condition with no evidence of improper
overcurrent protection, conductor insulation failure or deterioration,
and with no improper connections or splices. The electrical inspector
must inspect all repairs, alterations, or extensions to the electrical
(2) The insulation must meet Class I specifications as identified
in the Uniform Building Code, with a flame spread factor of twenty-five or less as tested using ASTM E84-81a. Foam insulation may not be used with knob-and-tube wiring.
(3) All knob-and-tube circuits must have overcurrent protection
in compliance with NEC Table 310.15 (B)(16), 60 degree centigrade,
Column C. Overcurrent protection must be either circuit breakers or
Type S fuses.
[Statutory Authority: Chapter 19.28 RCW, RCW 19.28.010 and 19.28.031.
WSR 17-12-021, § 296-46B-394, filed 5/30/17, effective 7/1/17. Statutory Authority: RCW 19.28.006, 19.28.010, 19.28.031, 19.28.041,
19.28.061, 19.28.101, 19.28.131, 19.28.161, 19.28.171, 19.28.191,
19.28.201, 19.28.211, 19.28.241, 19.28.251, 19.28.271, 19.28.311,
19.28.321, 19.28.400, 19.28.420, 19.28.490, 19.28.551, 2002 c 249,
chapters 34.05 and 19.28 RCW. WSR 03-09-111, § 296-46B-394, filed
4/22/03, effective 5/23/03.]

But again let’s be clear: Article 394 of the NEC leads many to assume that the issue with knob & tube wiring is heat. I challenge anyone here to describe the physics where knob and tube, with soldered junctions, can generate any significant heat. If you think it’s heat, you’re not correctly identifying the more significant and important issues.

Please don’t assume the practice and code in your areas are the only valid ones.

Note the bit about Type-S fuses – solid advice.

That would be a huge challenge in my area, if not impossible, and I would imagine it would also be in most other parts of the country, with 75 plus year old wiring. Very few electrical contractors are going to put their name on that.

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Again, it’s entirely common here. No, not every licensed C10 will do it, but there are plenty who will. They likely pick up plenty of work from stuff they find, to cover the relatively modest inspection fee. I encourage the client to bring a modern circuit (or two) into the kitchen, because who knows when someone will get a hankering to join the InstantPot revolution, or get a George Foreman Grille.

Nothing in the code mandates that the 75 year old electrical be upgraded in the first place. There’s zero heat problem. It’s been working for 75 years, and it’s made of copper which won’t go anywhere. The insulation is hardly if at all needed given the tubes & knobs.

So it’s really about do you prohibit these homes from getting insulated… and raising awareness of k&t (locally there’s a sign required warning future attic spelunkers of the burred live conductors).

In reality the homeowner’s insurance company may have the final say on K&T or aluminum wiring in a home. In my reports I suggest the buyers check with their insurance provider if they have K&T or aluminum wiring.


Thank you Randy. Great graphics.

Some insurance companies balk at each of those.

(Surprisingly, I have yet to hear a credible story about insurance companies and Federal Pacific breakers.)

In some cases insurance companies say they don’t do K&T, but their actual underwiring rules are they won’t insure houses with fuses. I encountered one situation where K&T had to be removed “90%”, though nobody said if that was 90% of the amps or 90% of outlets or… what exactly did that mean **?


** Note the 90% person replaced everything except the lighting circuits, where cost is highest and the tangible benefit the lowest…