Buried Knob & Tube

DSCF0011 (Small).JPGI know that knob and tube wiring should not be covered. Even if it’s like this situation?? Can anyone explain the real purpose why it can be covered?? One other thing, what is the approved method for splicing into knob and tube with newer wiring. (special wire nuts, in a juction box, etc.)

Your picture is a textbook example of nonconforming insulation of cavities containing concealed knob and tube wiring. Knob and tube is reliant on being in free air to dissipate heat. When the cavity is insulated, the conductor insulation will degrade, which can present a hazard.

In years past, taps and section replacements on K&T were made with an “End-O” fitting on type AC or type NM cable. In modern times, we must bring the K&T into a more traditional junction box, with the appropriate conductor supports, loom, and proper fittings. Read about that procedure here.

Thank you Marc, so when I see any insulation around k&t, I should call it out as wrong and recommend insulation be removed correct?

I’m not sure if you were directing that question toward me or not. In case you were, I’d have to say that I’m not qualified to answer that. I’m not a home inspector. It is my opinion, as an electrician, that either the insulation or the K&T should be removed. Depending on how long the insulation has been in place, the damage may already be done. I see it’s Miraflex type stuff, so it’s not terribly old. I’m just not sure if you’re permitted (as an HI) to recommend which remedial action; ie., removing the insulation or removing the K&T. Surely, someone else will chime in with that information.

I note the new junction box in the photo. Is the old K&T still in use?

If one recommended a qualifed electrician evaluate the K & T wiring, the electrician could make the call after seeing the condition of the wiring.

You always post helpful, thoughtful information. Your responses to the point, are not arrogant, like we should already know the answer. You don’t quote code ad nauseum, or try to sell us anything. I have benefitted from your efforts, and I’m sure others have also. Thank you.


Ditto Marc,… but Stuart beat me to it! Was thinking the same thing,… very helpful electrical post by you, along with the likes of Paul A. and others. Thanks!

I was surprised to learn that opinion on this question is less cut-and-dried than I had supposed:


Well spoken.
My exact sentiments as well.
Marc does us all well with his comments.

Michael Thomas, Great article, lots of info. Please just tell me in your words what you would state if found K&T wiring with any type of insulation around it. Thanks to all and Marc I agree with the good things being said also.

Usually the addition of insulation is always a case of being done many years after the intent of the wiring method. The client probably paid a fortune in heating and cooling bills and simply had someone come in and insulate which defeats the purpose of the K & T Wiring to be effective as it again as marc stated is designed to be in a free air space.

In most cases today we simply see the addition of junction boxes to make these changeovers and corrections and more so making it easier with the use of Nonmetallic boxes.

Yes, you should call out K & T all the time for exaluation but when you start to see K & T in attics and other places and jamed in insulation it is very important that your reports 1.) reflect the environment you see the wiring and type of wiring 2.) The condition that makes the wire a problem

Here is some info on K & T also in HI Translation: http://www.allaroundthehouse.com/lib.def.na10.htm

“Please just tell me in your words what you would state if found K&T wiring
with any type of insulation around it…”


Further evaluation of Knob and Tube Wiring: The Knob and Tube Wiring (K&T) [at location] is surrounded by [type] insulation. Since 1987, national standards have prohibited the installation of new insulation in contact with knob-and-tube wiring. Some authorities are concerned that insulation surrounding K&T wiring may raise its operating temperature, possibly shortening its service life and/or increasing the likelihood hazardous conditions may develop.

Additional information regarding these concerns is available at:


Where the K&T wiring is surrounded by insulation, I was not able to visually observe the extent or condition of the wiring. In houses where K&T wiring is present, I often observe defects in the condition, installation or connections of the wiring. In my opinion and experience it is prudent to assume that such defects may also be present in sections of K&T wiring I cannot visually inspect because they are obstructed by insulation.

Where K&T is present individual exposed wires are strung between ceramic insulators. As the wires are unprotected they are subject to damage if disturbed. The risk of such damage is increased when the wiring is not visible (as for example where it is installed under or behind insulation). In addition, K&T wiring is frequently incorrectly modified and/or incorrectly connected to newer wiring. Where K&T wiring has been damaged or incorrectly modified or connected, it poses a risk of electrocution and/or fire.

I recommend that all K&T wiring be inspected by a qualified, licensed and insured electrician, and that you obtain their advice as to 1) what portion (if any) of the insulation covering the K&T wiring should be removed to allow its inspection and/or reduce its operating temperature, and 2) what modifications (if any) should be made to this wiring to increase its serviceability or safety.

I would say this whether it had insulation around it or not:

“The residence is wired with suspect knob-and-tube wiring, which was commonly installed prior to 1950. It is ungrounded and over time the wire’s insulation may become brittle and fall apart or wear thin, resulting in exposed conductors and a risk of shock and/or fire. The hazard is increased by covering it with insulation (a common practice), and incorrectly tapping new wiring into it. The wiring should be evaluated by an electrician and certified as being safe or replaced.”