Knob-and-Tube Wiring: Years Used and Insurance Problems

What knob-and-tube wiring is

Knob-and-tube was the first standard way to wire American houses. Instead of running cables inside a single jacket, it used individual insulated copper conductors strung through the framing on porcelain hardware. The hot and the neutral wires run separately, usually several inches apart, so the heat each one gives off can escape into the open air around it.

According to InterNACHI, the system has three visible parts: porcelain knobs nailed to joists and studs that hold the wires in place, porcelain tubes where the wires pass through drilled lumber, and a cloth or rubber sleeve called loom where wires enter a fixture, switch box or wall cavity. There is no ground conductor.

Years used

InterNACHI dates common use from about 1880 to the 1940s (source). In practice that means:

Year built	How likely	What it means
Before 1930	High	Original wiring was very likely knob-and-tube. Ask what has been replaced.
1930 to 1949	Medium	Mixed: some homes were built with it, others with early cable systems.
1950 and later	Unlikely as original wiring	Can still appear where an older house was added to or partly rewired.

These ranges describe what was typical, not what is in a particular house. Many older homes were rewired decades ago, and many others were only partly rewired, which leaves live knob-and-tube circuits feeding a few rooms.

Why it worries inspectors and insurers

The wiring itself is not the whole problem. The risks come from age, from what has been done to it and from what now surrounds it. InterNACHI lists the main issues:

• No ground wire. It cannot safely serve three-prong appliances, and adapters or ungrounded three-prong outlets hide that fact.
• Insulation around the wires. The system relies on open air to shed heat. Loose-fill or foam insulation packed around the conductors traps that heat. The article notes that the 2008 National Electrical Code prohibits covering these conductors with loose, rolled or foamed-in-place insulation, although local rules vary.
• Brittle insulation and sagging wires. After many decades the rubber and cloth insulation can crack or flake off.
• Amateur splices and modifications. Because the wires are easy to reach in attics and basements, they were often spliced into newer cable, sometimes with tape instead of proper connectors.
• Oversized fuses. Fuses rated too high for the wire blow less often, which lets the conductors overheat.

Insurance follows from those risks. InterNACHI reports that many insurers refuse to cover homes with knob-and-tube because of fire risk, and that some make an exception when an electrical contractor certifies the system as safe (InterNACHI). State insurance regulators also list the condition and age of wiring among the things that affect premiums (Connecticut Insurance Department).

How to tell if a house has it

You can often spot signs of knob-and-tube without touching anything. Look, do not handle: if any wiring looks old, leave it to an electrician.

• Attic and unfinished basement: white or brown porcelain knobs nailed to joists, with single wires running between them.
• Porcelain tubes poking through joists where wires pass through the wood.
• Cloth-covered single wires rather than flat plastic-sheathed cable.
• Two-prong outlets throughout older rooms. They do not prove knob-and-tube, but they are a reason to ask.
• A fuse box instead of a breaker panel, or a breaker panel with old cloth-covered wires entering it.
• Insulation covering wires in the attic, especially loose fill. If the house was built between 1919 and 1990, ask whether that loose fill could be vermiculite, which EPA advises you not to disturb (EPA).

Visible knob-and-tube in an attic does not tell you whether it is still energized. Some systems were disconnected when the house was rewired and simply left in place. Only an electrician can confirm what is live.

Questions to ask

The home inspector

• Did you find any knob-and-tube, and is any of it still energized?
• Are there splices between knob-and-tube and newer cable, and how were they made?
• Is any knob-and-tube buried in insulation?
• Are there ungrounded three-prong outlets?

InterNACHI tells its inspectors to disclaim knob-and-tube in reports and refer clients to an electrician, so expect a recommendation rather than a verdict.

The seller

• When was the house last rewired, and by whom? Was it permitted?
• Is the rewiring complete or partial? Which rooms are still on old circuits?
• Has an insurer ever asked about or refused to cover the wiring?

The insurer

• Do you write policies on homes with active knob-and-tube wiring?
• If not, what documentation do you need, for example a licensed electrician’s evaluation or proof of rewiring?
• Does partial knob-and-tube count the same as a full system?

Ask before you are committed to the purchase. Coverage problems found late can delay closing.

What to do if you find it

• Get a licensed electrician’s evaluation. InterNACHI recommends having a qualified electrician evaluate the system. This is not a do-it-yourself job.
• Avoid overloading old circuits and do not rely on them for kitchens, bathrooms, laundry or outdoor use, as InterNACHI advises.
• Talk to your insurer early so you know what they require.
• If you are buying, get the electrician’s assessment and an estimate before the inspection contingency ends, and use it in negotiation.
• Plan any insulation upgrade with the electrician. Adding attic insulation over live knob-and-tube is one of the situations codes restrict.

This guide does not give instructions for electrical work. Any repair, replacement or disconnection should be done by a licensed electrician in your state. See our Disclaimer.

Knob-and-tube and other old wiring

Knob-and-tube is not the only wiring that worries insurers. Homes wired between 1965 and the mid-1970s may have aluminum branch-circuit wiring, which the CPSC links to overheating at connections (CPSC). The decoder for a 1970 house shows how the risks shift from one era to the next.

Partial rewiring: the most common situation

Many older houses are neither fully original nor fully rewired. A kitchen was redone, a panel was swapped, an addition was wired with modern cable, and the rest of the house still runs on the old circuits. That mix is where the questions get harder, because a modern-looking panel and a few three-prong outlets can make a house look updated when it is not.

• Ask the electrician to identify which circuits are still knob-and-tube, room by room.
• Ask whether any old circuit feeds a kitchen, bathroom, laundry or outdoor outlet, the locations InterNACHI says should not rely on it.
• Ask whether old and new wiring are joined anywhere, and how those connections were made.
• Keep the electrician’s written findings; insurers often want documentation, not a verbal opinion.

Planning a rewire in an older house

If you decide to replace the wiring, the work is planned with a licensed electrician and usually needs a permit. Topics worth raising early:

• Access. Plaster walls, finished ceilings and tight attics affect how the new cable is routed and how much patching follows.
• Phasing. Some owners rewire in stages, starting with the circuits that matter most for safety and insurance.
• Insulation. Once the old wiring is out of service, insulating the attic becomes simpler. Coordinate the order of work.
• Other old materials. Opening walls in an older house can disturb lead paint or materials that may contain asbestos. Ask how the contractors will handle that before work starts, and see our guides on lead paint and asbestos.

Common misunderstandings

• “It has worked for 100 years, so it’s fine.” The design can be sound, but age, insulation and decades of modifications are what create the risk.
• “Three-prong outlets mean it was rewired.” Not necessarily. Outlets can be swapped without adding a ground wire.
• “The inspector would have said if it was dangerous.” Inspectors typically flag it and refer you to an electrician; they do not certify it.
• “It’s all disconnected.” Possibly, but only an electrician can confirm that.

Related guides

• Home insurance for older homes: what raises premiums
• Polybutylene pipe: years installed and insurance issues
• What year was lead paint banned?
• All Electrical & Heating guides

Sources

• InterNACHI: Knob-and-tube wiring
• Connecticut Insurance Department: What affects homeowners premiums
• CPSC: Repairing aluminum wiring (Publication 516)
• EPA: Asbestos-contaminated vermiculite insulation