The current consensus that K&T should not buried in insulation comes mainly from two theories;
- It can not dissipate its own heat
- deteriorated wire insulation and connections outside of electrical boxes pose unseen hazards to those who might be working in the space.
Lets look at the first one. “It can not dissipate its own heat.”
More accurately, It was not designed to operate burred in insulation. While this may seem like semantics, it is important to distinguish between the two statements. The difference is the original design and intent of use.
The statement “designed to operate in free air” suggests that it was purposely designed that way. The designers gave thought about it being buried but in the end developed a product that that either they, or though testing, decided it should not be buried. This is unlikely since insulation was not used very much back then. At least by today’s standards.
The statement “not designed to be buried in insulation” means just that. It was not considered in this application. It may work just fine who knows. My wife’s cooking was not designed to fill the kitchen garbage can but it works just fine in doing it
The load on the circuit may have changed over the years as people tapped into it for more power how ever if the over current protection device is sized accordingly there should not be any heat dissipation problems. After all, if we can bury a Romex cable (with two conductors running side by side) in insulation, why would that cause less heat than the same gauge of single K&T wire. It won’t. In fact in theory it should be more because of the bundling of the two wires into a single cable. The difference comes in the wire insulation. The wire insulation used in Romex has been tested and marked that it will withstand a certain temperature. What is the temperature that the rubber wire insulation used on K&T will withstand? Is it still the same after decades of use? Unknown.
It is for this reason that I prefer to use the statement “not designed to be buried in insulation”. This, in conjunction with not being able to fully see an aging wire system for inspection on a regular basis are good enough reasons in my opinion to just replace it.