Insulating over knob and tube wiring


Old 10-14-15, 12:27 PM
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Insulating over knob and tube wiring

I've got a 3 story Victorian with an unfinished attic. The attic has very little insulation and I'd like to build some storage platforms and blow in insulation to help with heating and cooling.

There is a mix of K&T and Romex that runs beneath the floor boards in the attic and feeds the second floor.

Is it OK to blow in over these floor boards? Do I need to replace all the K&T first?

Old 10-14-15, 12:45 PM
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According to the electrical code you are not to insulate over K&T.
Old 10-14-15, 12:46 PM
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Right, you can't directly cover the K&T because the insulation would come into contact with the wire insulation. But that's not the issue or question.
Old 10-14-15, 01:00 PM
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because the insulation would come into contact with the wire insulation
Are you sure that's the reason? I would think it would be heat buildup.
Old 10-14-15, 02:30 PM
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The unfortunate side of that question is, once you bury that K&T no one is going to revisit the option in the future. Blown-in insulation doesn't do well when moved and replaced, so now is really the best time.

Old 10-14-15, 02:49 PM
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The code book doesn't really give us reasons. Reasonable explanations I've heard are preventing heat build-up which breaks down wire insulation, preventing sparks from igniting the insulating material, hiding the conductors which then become a trip and/or electrocution hazard for a person crawling through the attic.

Ultimately it sounds like a nuanced enough situation that the local AHJ would need to render a verdict and probably your home insurance company too.

I can share a tragic anecdote about a family in my home town whose house burned down the day after an insulation company blew cellulose into the attic. The allegation is that they blew cellulose over/into some electrical devices which caused the fire, don't know what kind. The contractor's insurance company and the home owner's insurance company have been fighting over this for almost three years, while the family still does not have a liveable home and is being supported by extended family and charity.

If it was my house, I'd replace the K&T prior to insulating whether specifically required to or not.
Old 10-14-15, 03:07 PM
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Ugh. That's heartbreaking IB.

I'm sure there's more than once such story.
Old 10-14-15, 07:43 PM
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On the other hand...

I grew up in a house that had knob & tube wiring. My parents bought this house about the time I was born and the first winter it cost, according to what my daddy told me, about $250 to heat. That so infuriated him that he had the entire house insulated with blown-in mineral wool. He said the company salesman tried to dissuade him from doing the walls, stating it was not cost-effective, but my daddy was adamant. He also had interlocking metal weatherstripping added to all the windows and doors. The next winter the cost to heat was about $50 and continued to be about that price until the oil embargo of 1973. (Lots more to this story but I will spare you.)

The house originally had FOUR 120 volt branch circuits. The garage (detached, two-car) was only 120 volts, on the same circuit as the kitchen and all was wired with #14 rubber/linen insulated wire. Because his table saw had such a high starting current he had installed a 30 ampere fuse in this circuit. I'll add that my mother had a large electric waffle machine and a large electric frying pan that she used fairly often on this same circuit.

In the attic over the kitchen the insulation was a bit deeper than the height of the ceiling joists so much of the wiring was at least semi-buried in the insulation. Over the years my daddy and I added several more circuits, with type NM cable and slowly de-energized much of the K&T. Still, there was a fair amount of the K&T that remained, that going down the walls to the light switches was fully buried in the insulation. Eventually, we installed a new circuit breaker panel (without changing the Service wiring or meter, which was 60 or perhaps 100 ampere capacity) all without any permitting or official inspections. We ran #8 type RHW single conductors from the CB panel up through the wall into a storage closet and with about ten inches of the individual wires showing up into the attic where we ran them across the tops of the ceiling joists to the same point where the original wires went overhead to the garage. Went through the wall of the house and then to the garage. We DID add a mast to the garage so the minimum clearance for these wires was at least ten feet above the ground. Added a sub-panel in the garage and had a 70 ampere CB in the house because the chart in the Sears catalog stated that #8 type RHW could carry 70 amperes.

Now of course all of this was contrary to code as that 70 ampere rating only applies to individual conductors in open air but it was years later that I learned that. Also, the individual conductors needed to be in conduit except where actually in the open air between the house and the garage. But, it was like that from about 1965 until the house was demolished in maybe 2000 without any problem. Even the #14 K&T wiring was still in good condition, despite having been overloaded and buried in insulation when I removed it. The only places where the insulation had deteriorated was in the ceiling boxes of overhead lighting fixtures that had been severely overlamped, 150 watt bulbs in fixtures rated for maybe 75 watts max. One thing I DID do right was that I connected what was left of the K&T (mostly just the lights in the upstairs and the front porch and hall lights) to 15 ampere circuit breakers and the rest of the newer wiring to the appropriate sized circuit breakers.

Now this is NOT to condone burying K&T in insulation nor to advocate doing any wiring without permit or inspection. All I am doing is telling you that under certain circumstances that K&T wiring that is covered with insulation is NOT an immediate danger nor is it going to fail with disastrous results. It IS best to remove it in favor of more modern wiring methods when at all possible.

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