Knob and tube wiring: how to tell, whether it is safe, and what it means for insurance and a sale

What you will see in the attic

Knob and tube was the normal way to wire a house until about 1950. Two single wires, one hot and one neutral, run a few inches apart. Porcelain knobs hold them against the framing, and porcelain tubes protect them where they pass through a joist. The wire is wrapped in cloth or rubber that dries out and cracks with age.

Where to look:

• Attic and crawl space. Knobs nailed to joists, with wires strung between them, are the giveaway.
• Basement or garage ceiling. The same pattern shows up on open beams.
• Outlets. Two-prong outlets with no ground fit the era. Newer outlets can still be wired to old circuits, so the plug style does not prove anything.
• The panel. Old fuse boxes, or a panel with only a few circuits for the whole house, often go with it.

Do not touch or cut the wires, and do not work around them in the attic. Have a licensed electrician confirm what you have. Many homes are mixed, with old runs left in place beside newer cable. See how to verify an electrician’s license before you hire.

Why insulation is the main hazard

Knob and tube was designed to shed heat into open air. Cover it with insulation and the wire can overheat. NEC 394.12 (the code carries this section, and California’s 2025 Electrical Code is based on the 2023 NEC) does not allow concealed knob and tube in hollow spaces of walls, ceilings or attics that are insulated with loose, blown-in, rolled or foamed-in material around the wires.

That is why a home energy upgrade is a common trigger. A crew that blows insulation into an attic can bury live wire. Other risks come from age and changes over the years: brittle insulation, splices added later, and fuses or breakers upsized to stop tripping. The rewiring hub and the knob-and-tube replacement page cover the technical side.

Existing knob and tube is not banned outright. It cannot be extended, and some cities add rules, such as a certification by a licensed electrician and warning signs where insulation is allowed. Ask your building department.

Insurance

Insurers decide this, not the code. Some may ask about old wiring, charge more or decline to cover the home. Rules differ by company, so ask your agent in writing.

If a standard insurer will not cover the home, the California FAIR Plan is the fallback. The Department of Insurance describes it as the state’s insurer of last resort, run as a private association of insurers. Coverage differs from a standard policy, so compare before you rely on it.

Ask your insurer these in writing: does the policy cover a home with knob and tube, will it renew, and what would change if the wiring were replaced? Keep a copy of the electrician’s report and permit record.

Selling or buying a home with it

A buyer’s inspector will report knob and tube, and a lender usually needs proof of insurance to close. If the insurer will not write a policy, the sale can stall. Sellers have three options: replace the wiring before listing, give a credit for it, or disclose it and sell as-is to a buyer who plans to rewire. The home sale inspection guide lists what inspectors check.

Get a written scope from an electrician before you make a decision. It should say which circuits are live and which are dead, so no one has to guess.

Partial or full replacement

A partial rewire replaces only the circuits still on knob and tube. It works when the rest of the house has newer cable. A whole-house rewire makes sense when most circuits are old, when the panel is also outdated, or when walls are open for a remodel.

Either way, the goal is no live knob and tube left. A partial job that leaves one run in the attic keeps the insurance problem. Ask for a circuit map before and after.

Cost, permit and inspection

As of October 2026, the typical SoCal range for replacing knob and tube in a home is $8,000 to $15,000, plus patching, and a whole-house rewire is $8,000 to $20,000. Those figures come from contractor price pages and are shown with their sources on the knob-and-tube cost page and the whole-house rewire cost page. Patching drywall and paint is extra, and plaster walls cost more than drywall.

Replacement is permitted work, and the city inspects the new wiring before walls are closed. See electrical permits in Southern California.

Where SoCal homes are old enough

The Census Bureau’s American Community Survey reports when homes were built, and the numbers show how much older housing a place has. These are the share of homes built before 1950 in some cities, from the figures this site uses:

City	Built before 1950	Median year built
South Gate	47%	1952
Pasadena	36%	1961
Burbank	35%	1959
Long Beach	34%	1959
Alhambra	34%	1960
Inglewood	32%	1960
Los Angeles	28%	1965
Riverside	10%	1976

A pre-1950 date does not mean a home still has knob and tube. Many were rewired. It means a check is worth the time. In newer places such as Irvine, the chance is far lower.