Guide

Knob-and-tube wiring: what it is, and what insurers think

Identification first, then the underwriting question — because for most people reading this, the letter from the carrier came before the curiosity.

Updated September 6, 2026 · Data as of Sources retrieved 6 September 2026

Written by HyreElectrical Research Desk Primary-source research and fact checking

11.9% of US homes built pre-1940 the installation era — not a count of surviving knob-and-tube

None federal count of homes still wired this way no agency publishes one; we do not invent one

2 of 2 carrier guides reviewed list it ineligible named, dated underwriting documents — a sample, not a market survey

The short version

Knob-and-tube is an open wiring method used in American homes from roughly the 1880s to the 1940s: single insulated conductors run separately through porcelain knobs where they cross framing and porcelain tubes where they pass through it, relying on air around the conductor to carry heat away. It is not illegal, and existing installations are not required to be removed. NFPA 70 still carries it as Article 394, and the model code permits existing concealed knob-and-tube to remain. What it is not is grounded — there is no equipment grounding conductor, so no three-prong outlet on that circuit is doing what its third slot implies. The practical problem is usually insurance, not code. Multiple carriers list knob-and-tube among ineligible risks outright, and unlike aluminium wiring — where at least one carrier publishes a remediation route — the guides reviewed here state no path back to eligibility. The other live issue is thermal insulation: NEC 394.12 generally prohibits burying knob-and-tube in loose, rolled or foamed-in insulation, though at least one state publishes a conditional exception. HyreElectrical does not perform, supervise or warrant electrical work.

What it actually is

16,917,912 American housing units — 11.9 per cent of the stock — were built in 1939 or earlier. Including the 1940s brings it to 16.4 per cent. Source fact: US Census Bureau, American Community Survey 2023 5-year estimates, table B25034. HyreElectrical calculation: the percentages are ours, computed from those counts.

Knob-and-tube is a two-wire open wiring method. The hot and neutral conductors are not bundled in a common sheath the way they are in the cable used today. They run as separate single conductors, spaced apart, held off the framing on porcelain knobs and threaded through porcelain tubes wherever they pass through a joist or stud.

That separation is the design, not sloppiness: the method depends on free air around each conductor to shed heat. Almost everything that later goes wrong with knob-and-tube follows from something interfering with that assumption.

Knobs: Porcelain insulators nailed to framing that hold a conductor away from the wood as it runs along or across a member.
Tubes: Porcelain sleeves set into a drilled hole so a conductor can pass through a joist or stud without touching it.
Loom: Flexible insulating sleeving over the conductor where it enters a box or fixture, or where it runs in a tight space.
No equipment ground: The method has two conductors and no grounding conductor. This is the single most consequential fact about it, and it does not change with age or condition.
Article 394: The NFPA 70 article covering concealed knob-and-tube. Its continued existence is why an installation is permitted to remain rather than being outlawed outright.

Read that chart carefully. It shows when homes were built, not how they are wired. Most homes of this age were rewired decades ago, many in stages. No federal source counts homes with active knob-and-tube, so neither do we.

How to recognise it, without opening anything

Knob-and-tube is visible in the places where a house is unfinished — an open basement ceiling, an unfloored attic, the back of a closet in an ell. You do not need to remove a cover, a panel front or a fixture to look, and you should not.

White or brown porcelain knobs nailed to joists and studs
Roughly spool-shaped, with a nail through the centre and the conductor sitting in the groove.
Porcelain tubes in drilled holes through framing
A short ceramic sleeve where a wire passes through a joist. This is the clearest single indicator.
Two separate conductors running parallel, several inches apart
Not a single grey or white jacketed cable. Two wires taking their own paths is the signature.
Cloth or rubber conductor insulation, often stiff and darkened
Modern thermoplastic insulation stays flexible. Fabric-covered rubber does not.
Two-prong outlets throughout an older part of the house
Consistent with an ungrounded system, though it is evidence, not proof — other ungrounded methods look the same at the outlet.
BAD SIGN — soldered splices hanging in open air, taped
Original splices were soldered and taped in the open. Any splice you can see should be in a box today.
BAD SIGN — knob-and-tube disappearing into blown-in insulation
This is the condition NEC 394.12 addresses, and the one most likely to matter to both an inspector and an underwriter.
BAD SIGN — modern cable spliced onto original conductors mid-run
Later modification by non-professional hands is a more common source of trouble than the original work.

Where to stop. Scorching, a smell of hot plastic or varnish, lights that dim and brighten, or a breaker or fuse that will not hold is no longer an identification question. Stop and have a licensed electrician determine the cause.

The insurance problem, which is usually the real question

Most people arrive at this subject because a carrier raised it — at renewal, at purchase, or after an inspection report. So it belongs here rather than at the bottom.

Source fact. The two underwriting documents below were read directly. Both name knob-and-tube among ineligible risks. The distinction worth noticing is against aluminium wiring: one of these same carriers publishes a remediation route for aluminium — properly pigtailed, with proof from a licensed electrician — and neither publishes an equivalent route for knob-and-tube.

HyreElectrical analysis. Two guides are a sample, not a market survey, and we are not going to tell you what "most insurers" do from a sample of two. What the sample does establish is that outright ineligibility is a real published position rather than a contractor talking point, and that the absence of a stated remedy is a meaningful difference from how the same documents treat aluminium.

Carrier	Document and date	What it says	Stated route back
Orion180 Insurance Company	Texas Flex Program Underwriting Guide, Homeowners Program 13 January 2025	Lists “Knob and tube wiring” among ineligible risks, alongside aluminium wiring, double-tapped wires and named unsafe panel brands.	None stated. The risk is ineligible for the programme.
Homeowners of America Insurance Company	Texas Homeowners underwriting guidelines, ineligible risks no revision date published on the page; retrieved 6 September 2026	Lists as ineligible “Risks with Aluminum or knob and tube wiring.”	The published remediation route is stated for aluminium, not for knob-and-tube.

Source facts, quoted from each carrier’s own underwriting guide, retrieved 6 September 2026. Underwriting rules vary by carrier, programme and state, and they change.

Ask your own carrier in writing what they require and what evidence they accept, before commissioning any work — a scope built to satisfy an unwritten requirement cannot be enforced later. See the panels and wiring insurers decline for the wider picture.

The four ways it actually fails

Correctly installed, undisturbed, and still carrying the load it was designed for, knob-and-tube is not the emergency it is often described as. Three of the four failure modes below are things done to it after installation, not properties of the method — the distinction a good electrician assesses and a generic article flattens.

1. Insulation contact

The method needs free air to shed heat. Bury it in blown-in cellulose, rolled batts or foam and the conductor runs hotter under the same load. This is what NEC 394.12 addresses, and it is the most common serious finding, because attic insulation was retrofitted into millions of older homes by people with no reason to think about the wiring under it.

2. Overfusing

These circuits were sized for the loads of their era. Replacing a blown 15-amp fuse with a 20- or 30-amp one — historically easy in a screw-base fuse panel — removes the protection the conductor depends on while leaving everything looking normal. Washington’s own rule addresses this directly by requiring Type S fuses or breakers and protection per the 60 °C column of NEC Table 310.16.

3. Deteriorated conductor insulation

Rubber-and-cloth insulation embrittles with heat and age. It tends to fail first where it has been hottest and most handled: at fixtures, at switches, and in the last few inches entering a box. A system can be sound in the open runs and unsound at every termination, which is why a condition assessment is not a glance from the basement.

4. Later modification

A century of extensions, splices and "just this one outlet" — modern cable spliced to original conductors, splices left outside boxes, grounded three-prong receptacles fitted to an ungrounded circuit so the plug fits. HyreElectrical analysis: this is the failure mode most often mistaken for the original wiring being dangerous, because the mess and the method share a cavity.

Insulation over knob-and-tube: the rule, and the exception nobody mentions

Source fact. Under NFPA 70, Article 394.12 generally does not permit concealed knob-and-tube in hollow spaces of walls, ceilings and attics where the space is insulated with loose, rolled or foamed-in material that envelops the conductors. That is the rule almost every consumer page reports, and it is correct as a general statement of the model code.

What those pages leave out is that the model code is not the adopted code. States amend it. Washington has published a conditional pathway in its own administrative code — WAC 296-46B-394, filed 21 February 2024 and effective 1 April 2024 — under which loose or rolled insulation may be installed in spaces containing existing knob-and-tube provided every condition below is met. The text was read from the state legislature’s own site.

Condition	What Washington requires
Survey and certification	An appropriately licensed electrical contractor must survey the wiring and certify in writing to the department that it is in good condition — no improper overcurrent protection, no conductor insulation failure or deterioration, no improper connections or splices.
Inspection of any work	The electrical inspector must inspect all repairs, alterations or extensions to the electrical system.
Insulation specification	The insulation must meet Class I specifications as identified in the Uniform Building Code, with a flame spread factor of 25 or less tested to ASTM E84-81a.
Foam	Foam insulation may not be used with knob-and-tube. The exception does not extend to it.
Overcurrent protection	All knob-and-tube circuits must be protected in compliance with NEC Table 310.16, 60 °C column, using either circuit breakers or Type S fuses.

Source fact: Washington Administrative Code 296-46B-394, "Concealed knob-and-tube wiring", read from app.leg.wa.gov and retrieved 6 September 2026.

HyreElectrical recommendation. The Washington conditions are not permission anywhere else, and the general prohibition is not the last word in your own state. What governs your house is the NFPA 70 edition your jurisdiction adopted and how it amended it. Our NEC edition lookup is the starting point; the building department is the authority.

What to actually do about it

In order, and the first two cost nothing.

1
Establish whether you have it at all
Look in the unfinished spaces for knobs, tubes and separated conductors. Two-prong outlets alone are not proof. Photograph what you find.
2
Ask your insurer in writing what they require
Their answer determines the scope. Get the requirement and the acceptable evidence in writing before anyone quotes work — a scope built on an unwritten requirement cannot be enforced later.
3
Commission a condition assessment, separately from a quote
The party assessing the wiring is very often the party selling the rewire. Paying for an independent assessment buys you a scope you can hand to several bidders on the same terms.
4
Ask specifically about partial versus whole-house scope
Original knob-and-tube may be sound in open runs and unsound at terminations, or fine everywhere except an insulated attic. "Whole-house rewire" is sometimes the right answer and sometimes the easiest one to sell.
5
Confirm the permit and inspection before work starts
Rewiring is permitted work, and when you need an electrical permit sets out what the model code exempts and what unpermitted work does at sale and at claim time. The inspection is the only step where somebody with no financial stake in the job checks it — including, in Washington, the certification the insulation exception depends on.
6
Deal with the grounding question explicitly
Knob-and-tube has no equipment ground. Ask what the scope does about existing three-prong receptacles on ungrounded circuits, because leaving them is a hazard the rewire was supposed to remove.

Related: whole-house rewiring covers the work itself, and aluminium wiring covers the other wiring type carriers ask about — which, unlike this one, does have a published remediation route.

What we could not establish

How many American homes still have active knob-and-tube. No federal source publishes it. The Census records when a home was built, not how it is wired. We publish the construction-era figure and label it as a weak proxy rather than converting it into a count somebody will quote back as fact.

A national fire statistic isolating knob-and-tube. We did not find one that separates it from other pre-1950 wiring and from the later modifications made to it. Numbers of this kind circulate widely; we could not trace one to a source that measured what the sentence claims, so this page publishes none.

What most insurers do. Two named guides establish that outright ineligibility is a published position. They do not establish a market share, and we will not extrapolate one from them.

Verbatim NEC wording. NFPA 70 is copyrighted and the full text of Article 394 was not retrievable from a free primary source on the retrieval date. Article 394 is therefore cited by provision and characterised here, never quoted. The Washington conditions are quoted, because state administrative code is public law.

Questions

Is knob-and-tube wiring illegal?

No. NFPA 70 still carries concealed knob-and-tube as Article 394, and existing installations are permitted to remain — there is no code requirement to remove wiring that is in good condition and properly protected. What is restricted is burying it in thermal insulation, and what is effectively restricted in practice is your ability to insure it.

Do I have to rewire my house if it has knob-and-tube?

Not as a matter of code. In practice the decision is usually driven by insurance, by a condition assessment finding deteriorated insulation or unsafe later modifications, or by a plan to insulate the attic. Ask your carrier what they require in writing first — that answer sets the scope more often than the code does.

Can I put insulation over knob-and-tube wiring?

Under the model code, generally no: NEC 394.12 does not permit concealed knob-and-tube in spaces insulated with loose, rolled or foamed-in material enveloping the conductors. But the adopted code is what governs, and at least one state publishes a conditional exception. Washington permits loose or rolled insulation over existing knob-and-tube if a licensed contractor certifies the wiring in writing, the insulation meets a stated flame-spread specification, foam is excluded and every circuit is protected per NEC Table 310.16 at the 60 °C column. Check your own jurisdiction rather than assuming either version.

Why will my insurer not cover a house with knob-and-tube?

The underwriting guides reviewed for this page list it among ineligible risks outright. Notably, one of the same carriers publishes a remediation route for aluminium wiring — pigtailed, with proof from a licensed electrician — but neither publishes an equivalent route for knob-and-tube. Rules vary by carrier, programme and state, so the only answer that binds is your own carrier’s, in writing.

Is knob-and-tube grounded?

No. It is a two-conductor method with no equipment grounding conductor, and that does not change with condition or age. Any three-prong receptacle installed on a knob-and-tube circuit is telling the person plugging into it something that is not true, which is a specific thing to raise with an electrician.

Written and audited by

HyreElectrical Research Desk

Primary-source research and fact checking

We read the model code, the federal safety notice, the municipal fee sheet or the utility tariff ourselves, and we publish the figure with the document it came from and the date we retrieved it. Where a number cannot be traced to a primary source, we publish the shorter page and say what we could not verify. HyreElectrical does not perform, supervise or warrant electrical work. Authorship is organisational: this desk, not a named persona.

10: long-form launch pages in this layout
5: states with verified licence records
16,369: electrical companies in those records
74%: of the store is Florida — disclosed first, not footnoted

How this desk works

Primary sources only. Code statements come from NFPA 70 as the model code. Safety statements come from CPSC. Permit fees come from the city or county fee sheet. Utility charges come from the filed tariff. We do not cite a blog that cites a source; we open the source.
Our contractor store is five states, and 74% of it is Florida. Any figure built on that store is titled to those states and names the concentration in the first screen. It is not a national sample of electricians.
Load calculations on this site are a published simplification in the shape of NEC 220.82 — first 10 kVA at 100%, remainder at 40%, HVAC at 100%, EVSE at 125%. They are labelled as not a stamped calculation. A licensed electrician using the adopted edition does that work.
No national price for a panel upgrade is shipped. Labour rates, the adopted code edition, whether the utility is involved, and the state of the existing wiring move the invoice too far for a roundup to help. Cost intent lives on the cost page; this page explains the decision.
We do not perform electrical work, and we take no payment for placement, ranking or a favourable mention. Nobody buys a position on this site.

Data as of Sources retrieved 6 September 2026. Authorship on this site is organisational: the analysis belongs to the desk rather than to a named individual, and we do not publish credentials we do not hold. Our editorial policy sets out how we source, date and correct what we publish.

Sources & retrieval dates

National Fire Protection Association — NFPA 70, National Electrical Code, Article 394 (Concealed Knob-and-Tube Wiring) , Cited by provision, not quoted. Article 394 is the reason existing concealed knob-and-tube is permitted to remain; 394.12 is the provision restricting its use in spaces insulated with loose, rolled or foamed-in material. NFPA 70 is copyrighted and the full section text was not retrievable from a free primary source on the retrieval date. Retrieved 6 September 2026.
Washington State Legislature — WAC 296-46B-394, “Concealed knob-and-tube wiring” , Every condition in the insulation table: contractor survey and written certification, inspection of repairs and extensions, Class I insulation at flame spread 25 or less to ASTM E84-81a, the exclusion of foam, and overcurrent protection per NEC Table 310.16 at the 60 °C column using breakers or Type S fuses. Filed 21 February 2024, effective 1 April 2024. Retrieved 6 September 2026.
US Census Bureau — American Community Survey 2023 5-year estimates, table B25034 (Year Structure Built) , The housing-era figures: 142,332,876 total units, 16,917,912 built 1939 or earlier and 6,429,315 built 1940–1949. The percentages are HyreElectrical’s calculation from those counts. Retrieved 6 September 2026.
Orion180 Insurance Company — Texas Flex Program Underwriting Guide, Homeowners Program, dated 13 January 2025 , Lists knob-and-tube wiring among ineligible risks, with no stated remediation route. Retrieved 6 September 2026.
Homeowners of America Insurance Company — Texas Homeowners underwriting guidelines, ineligible risks , Lists “Risks with Aluminum or knob and tube wiring” as ineligible. A remediation route is published for aluminium and not for knob-and-tube. Retrieved 6 September 2026.

Get a condition assessment before you get a quote

The party assessing century-old wiring is often the party selling the rewire. An independent assessment gives you one scope to put to several bidders.

Find a local electrician What rewiring involves

HyreElectrical does not perform, supervise or warrant electrical work, and takes no payment for placement, ranking or favourable mention. Underwriting rules vary by carrier, programme and state and change without notice; nothing here is insurance or legal advice.