Knob & Tube -- Cellulose Insulation

richardonthegoOctober 28, 2011


I am in Los Angeles in a house that was built in 1921. It has knob and tube and is still running electricity that way. We've had a licensed electrician come and inspect all the wires and breaker box and retrofit all the plugs to be grounded. We asked him if cellulose could be blown in over the knob and tube in the attic and he said that'd be fine because the wires are still in good shape.

I called an insulation company with experience in knob and tube and they said it's fine, as well. Especially because cellulose is non-flammeable.

I still have reservations, though. Despite both the electrician and insulation company saying it's safe, I'm worried. A lot of forums say to replace the knob and tube completely.

Should I be? Has anyone else blown in cellulose over knob and tube?

We are planning on only blowing in the cellulose in the attic, not the walls.

Thank you!

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Ron Natalie

Absolutely NOT according to the NEC. This is specifically DISALLOWED by 394.14

(5) Hollow spaces of walls, ceilings, and attics where such spaces are insulated by loose, rolled, or foamed-in-place insulating material that envelops the conductors

Further, the handbook explains why:

Blown-in, foamed-in, or rolled insulation prevents the dissipation of heat into the free air space, resulting in higher conductor temperature, which could cause insulation breakdown and possible ignition of the insulation. Section 394.12 prohibits installation of knob-and-tube wiring in hollow spaces that have been weatherized.

    Bookmark   October 29, 2011 at 10:35AM
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Richard -

We're in the same situation (up in the Bay Area).

The California State Energy Code differs from the NEC in that it allows insulation over knob and tube provided a licensed electrician inspects the K&T and deems it safe.

You can access a copy of the relevant code at the link to an Energy FAQ site below (it's the second to last question on the "In-Home Work Requirements" section).

I'm still undecided. I'm pretty risk averse, but I also don't want to avoid making my home more energy efficient due to what seems to be (statistically speaking) a very slim chance of fire risk. Especially since I'm looking at some generous rebates for energy upgrades.

Here is a link that might be useful: California Electrical Code

    Bookmark   November 10, 2011 at 1:09PM
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May be I am missing something... how is your electrician upgrading all outlets to be grounded without rewiring them? And if he is rewiring them, why not get rid of the knob and tub all together? I would never live in a house with knob and tube. Who knows what condition non-visible wires are in... and what condition any hidden splices are in... insulating will only make rewiring more difficult. I'd rewire first then insulate.

    Bookmark   November 10, 2011 at 10:26PM
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"I would never live in a house with knob and tube."

you cannot see most of the wiring in a house.

Most of the K&T problems are caused by folks messing around with the K&T.

It tends to work just fine (even if the insulation is brittle) until someone messes with it.

Pre-WWII AC cable is often pretty bad also.
Before the requirement to solder plat the copper wire it reacted with the rubber insulation making it extremely brittle by with time.

A lot of the insulation will fall right off the wire if you move it to replace a switch or receptacle.
Even moving the cable can result in shorts forming.

    Bookmark   November 11, 2011 at 5:36PM
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I often keep the K&T around for that very reason - almost no chance of shorting with the wires so far apart. As long as it's run in an area not covered with insulation (as in a basement), where the entire run is visible and can inspect for good splices. NEC allows for extending existing knob & tube wiring, though not new runs.

    Bookmark   November 27, 2011 at 7:38AM
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'NEC allows for extending existing knob & tube wiring"
Just try to buy pieces.

    Bookmark   November 27, 2011 at 9:55AM
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Other provisions in the code make it impossible to extend.

Like grounding requirements.

It is more of a historical record tat allows K&T to be considered still in accordance wth the NEC in existing installations.

    Bookmark   November 27, 2011 at 9:57AM
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