Knob and Tube Wiring and Insulation

by Erik North on March 2, 2012

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Folks often call about energy audits part of the way through their thinking on a project. Sometimes the big project decisions have already been made; homeowners are just looking for final guidance.

On more than a couple occasions, those plans to blow cellulose over an attic flat are brought up short by live knob and tube wiring. But what is knob and tube wiring and why is insulation such a big deal that the electrical code forbids that it be in contact?

What is Knob and Tube Wiring?

Knob and tube wiring is exactly what it sounds like; there’s knobs, tubes and wires. Ceramic knobs are fastened to the building frame, spacing the neutral and hot wires away from the wall. These wires often pass through the joists and studs where they are protected from abrasion by ceramic tubes.

The wire itself was originally sheathed in a protective cloth impregnated with asphalt though this was later replaced with a rubber coating.

Knob and Tube Wiring and Insulation

One advantage of knob and tube wiring is how well it dissipates heat. The knobs, used for mounting the wire on beams and rafters, suspend the wires in air, allowing plenty of cooling air flow.

When installed correctly, knob and tube wiring is quite safe. However, we can count on one thing in a house over the course of 100 years: people will make changes. Folks want several new outlets, electricity for the flat screen, microwave, computer and 100 other gadgets. Splicing new wiring into the existing knob and tube wiring is incredibly common.

So you’ve added extra electrical load to a system designed in 1911. The electrical fuse keeps popping. Replacing with a fuse rated for a higher current solves the popping, but the overfused wires carry more current and operate at a hotter temperature. Not good.

You have an overfused electrical wire diffusing gobs of heat suspended in the air of your attic. Not a problem at all…until you bury it in insulation. D’oh!

The knob and tube wiring is strung from joist to joist in your attic. There’s no insulation so the homeowner blows in 20 inches of cellulose, covering these very hot, overloaded wires.

This creates a pretty serious fire hazard. It is severe enough that the National Electrical Code now prohibits insulation from being in contact with knob and tube wiring.

If you are in the planning phase of an insulation project and have knob and tube wiring, you may have to address it. Contact a local electrician. They can examine the installation, determine whether or not it is live and help you with your house project planning.

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