Knob and tube: heat's the main problem?

Knob and tube is old by now and may be threadbare and badly spliced, but that’s not something we’re going to track down as inspectors if it’s buried in insulation.
My understanding is that installing additional insulation to save on heating costs is now so common that knob and tube is often buried and so is unable to dissipate heat the way it was designed to.

You can call it out as an outdated but acceptable wiring system as long as it has no visible wiring defects but if you miss calling out buried K&T as a possible fire hazard you may be open to liability. Is this right?

I guess it is possible Kenton if you do not explain in your report that you can only inspect what you see. I do not do many pre 40’s homes so I do not worry too much about it :slight_smile:

Good Article posted by Paul next door about inspecting Knob and Tube but I didn’t see anything about heat being a concern. There are a lot of older houses in town here and it still pops up occasionally.
Of course heat’s a concern all over Phoenix

Here’s what I say…got it off this board somewhere. I forget who to credit.

“Knob and tube wiring is partially covered by insulation. This method of wiring was designed to be open to the air to prevent heat build-up. This wiring method should be evaluated by an electrician and certified as being safe or it should be replaced.”

394.10 Uses Permitted. Concealed knob-and-tube wiring shall be permitted to be installed in the hollow spaces of walls and ceilings or in unfinished attics and roof spaces as provided in 394.23 only as follows:

(1) For extensions of existing installations

(2) Elsewhere by special permission 394.12

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

(1) Commercial garages

(2) Theaters and similar locations

(3) Motion picture studios

(4) Hazardous (classified) locations

(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"

This may sound on the surface like symantics but “K&T was not designed to be burried insulation” would be more accurate.

“K&T was designed to be in open air” implies that the original electrical engineers that devloped K&T had this in mind . The fact that orginal installations were in free air is inhernt from the attempts to keep it away from combustible material.

“K&T was not designed to be covered in thermal insulation” implies that although this application was probably not considered during the orginal designing of the electricla system in may or may not work fine. A re-evaluation of the system with it covered in the insulation would be needed to determind if it would provide adequate service and saftey.

Also, in general, The K&T systems were not designed to carry some of the loads that we expect today. This can also increase the temperature.

FYI: Did you know that the desiel engine was NOT originally designed to run on fossil fuel?

Ontario Hydro has not condemned K&T. It seems to be an issue driven by insurance companies. Mind you if its exposed and damaged it would be wise to upgrade. I think that is just common sense.

As to diesels you can run them on Canola oil, french fry oil or vegetable oils.

That’s right Raymond. Peanut oil was accually the fuel the deiesel engine was oringally designed for.


I am sure others have posted this but I will elaborate.

The main problems with K&T is the following in my opinion

1.) Usually only # 14 AWG and so it has a capacity limit.
2.) As you stated it is many times embedded in insulation which defects the designed installation of the product, It needs to be FREE AIR to allow for heat removal.
3.) The rubberish style jacket tends to crack off and expose the conductor which again is a real issue since it is usually in the older insultation that is not flame-retardent.

My largest concern is the capacity to be honest with you and the effects overloading the circuit can have on the OTHER issues given, with the advent of many chances in our lifestyles the owners tend to bring in new things like computers and flour. lights and so on that create harmonics and increases the load value on the neutral which is a shared capacity neutral sometimes with many rooms…it is also usually # 14 AWG…now you have a neutral on a circuit that could be overloaded…and add the fact it can’t radiate heat from the wire because it is also now baried in insulation because the home owners wanted some storage space…yeah we have all seen this.

Now ofcourse we are not speaking of any “grounding” potential as well but that is not nearly as much of MY concern as the previously mentioned issues…a GFCI on the circuit can add protection ( having nothing to do with grounding as you all should know ) but it does not solve the problem of potential overloading.

Usually in a K&T house we see maybe 2 receptacles per room, we suggest they upgrade as it is not able to be expanded…BUT we see extention cords and power strips all around…increasing the load and demand on a conductor that was not designed for it based on it’s layout…which is a huge common neutral in most cases…the poor neutral could have issues…even WORSE if the home is a 120V service…all on the same LINE will compound the issues on the neutral…now in an older 120/240V service…IF the circuits were done right you can reduce the issue a little…BUT all depends on the layout of the panel and hope it was done by someone who KNOWS the effects.

While K&T is accepted because the code is not retroactive…BUT it should always be called out and explained it is not able to handle the demands of our generation or the next generation…so it is a PAY NOW or PAY LATER issue…I prefer to have them PAY NOW…

I am not really sure heat is as big an issue with K&T as it would be with TW insulated paper Romex. If both were buried in the same insulation the Romex would be running hotter since the conductors are closer together and the main mechanism of failure would be failure of the TW insulation on the wire.
K&T is basically insulated by the knobs and the space between them.
I suppose at a certain point the wire could fail but that Romex would have burned up long before the copper failed. I suppose it would really come down to what type of thermal insulation was blown in the wall. If it was shreaded newspaper you have a hazard, no matter what kind of wire is there. If it was rock wool or glass, your K&T became more like MI cable.

David posts the rule from the NEC, and I wondered if he includes that information in his reports? I thought the HI was afraid of the NEC and advoids speaking to defects as violations.

Seems like some double standards here from both the US and Canadian sides. Roy?

What gives the NEC, or the SOP?

BTW, I think that the subject of the information related to K&T was covered long ago by others in the archives, and searching for that information will show that this wiring method is best left alone.

I also read where there are many issues in the attic spaces where the blown in insulation creates the problems now associated with possible fire hazards.

How many HI’s really crawl into the attic to have a look see?

I am not sure why everything we post here is considered something that would be put in a report…I give alot of code but no one ever says to include it in a report…we are all adults here so we will assume HI’s know what to include and what not to include…its all about education not telling someone how to write their report…

As for MY post…you did not see one bit of NEC in my explanation and I can QUOTE NEC Code with the best of them…lol

My openion Knob and tube has served its useful life .
Not adequate for todays home and should be upgraded immediately by a qualified person.

Roy Cooke sr …

As to code, you could always state along the lines that this method or installation does not meet current code, or is against code. Just don’t quote the code reference unless asked to prove otherwise post report.

If you have knowledge of code I see no reason why one would be out on a limb if stating code reference. If you are not sure don’t state its not to code you could say … this item “may not” meet code or item does “not appear” to meet code. But having said that you had better be certain you are not calling code for buildings that predate the current codes.

Most HIs I know inspect all attics where access is provided, including crawling to remote points provided it does not constitute a safety hazard or present the potential for damage.

All HIs should at least view the attic and components including wiring from an entry point.

How many HI’s really crawl into the attic to have a look see?

All of us.

It is my understanding that here in Ontario if knob and tube is uncovered during renovation it must be replaced.

Some one asked about the Ontario electrical code.
Here is an area that has a lot of frequently questions and the answers.
I think any one can ask a question that has not been asked .

The answer could take a while to be posted as this site I do not believe is monitored constantly .
Roy Cooke …


I open the attic hatch if there is one, and examine the visible areas with a spot light. I do not crawl around or try to go to hidden areas.

Naturally it would be good advice to upgrade when renovating.


I do attics, always have and will continue to do so. It is the one place other than the crawl space where you can see the bones or 'innnards" of a home. Have found many many problems that were not visible from the roof or inside the home. Yep, its hot as hades this time of year but you just have to suck it up and do what they are paying you to do.