National Electrical Installation Standards

Standards as High as Your Own

Monday, July 9, 2018


Good morning, An interesting question came to me today and I could not answer it. What about existing knob and tube wiring in a wall or ceiling that has been insulated?  Section 394.12(5) tells us it cannot be used, but the Code doesn't usually apply retroactively.  Can it be left in service? Should the insulation be removed around it, if possible?  What about wall cavities that have been blown full of insulation?  Was there ever a time when it could be in contact with insulation? The oldest code I own, the 1956 NEC, in Article 324, Section 3242, Use, indicates it "may be used in the hollow spaces of walls and ceilings."  Hollow would indicate empty, or uninsulated. Do you have any idea when the present language of 394.12(5) was added and what the substantiation for it might have been?  Is there any history of loss due to knob and tube wiring in insulation overheating?  Many older homes, not originally insulated, have K&T wiring in the attic and have since added insulation.  Is this a problem?

Thanks. Grant Hammett



Hey Grant thanks for your question. With the help of some friends it seems the words "in the hollow spaces in walls and ceilings" were added in 1928 in 502a. It also seems that hollow spaces would infer uninsulated. The Proceedings for that edition are available at:

but there seems to be no discussion about that change.

Code Making Panel 7 accepted proposal 7-2 for the 1987 NEC adding the specific wording about the wiring not being permitted if the spaces were insulated in 324-4. That proposal is on page 300 of the TCR (NEC-TCRA-1986-Articles 305 to 515.pdf) at:

There is no specific language about the wiring being required to be removed if no longer used but it also would seem prudent to no longer use it in that situation.

If any of our readers have additional information or evidence of problems with thermal insulation added in walls and ceilings that contain concealed knob and tube wiring, please send that in and we will share it.

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