Spot white ceramic knobs threading wire along the basement joists and you are almost certainly looking at knob and tube wiring — the standard in homes built from the 1880s into the 1940s. It is not automatically unsafe, but it is ungrounded, it ages badly, and it turns an insurance application into a negotiation. This guide covers how to identify it, why insurers balk, the buried-in-insulation hazard that matters most, rewiring costs, and how to word it in a report without overstating. Document old-wiring findings in HomeInspecto.
Old-wiring inspection guide
Knob and tube wiring: risks, insurance, and report wording
A wiring method from another century, still live in millions of homes. Here is how to read it, what makes it risky, and how to write it up so buyers and insurers trust the call.
- 1880s–1940sKnob & tube era
- 1940s–1960sGrounded cable arrives
- 1960s–todayModern grounded wiring
How to spot knob and tube
The system runs two separate conductors — a hot and a neutral — held away from framing by ceramic supports. Four tell-tale signs give it away in an attic or basement.
-
Ceramic knobs
White porcelain spools nailed to joists and studs, holding each wire a safe distance off the wood.
-
Ceramic tubes
Porcelain sleeves protecting the wire where it passes through a joist or stud.
-
Two separate wires
Hot and neutral run several inches apart, not bundled in a single sheathed cable.
-
No ground
Cloth or rubber insulation, two-prong outlets, and no equipment grounding conductor anywhere.
Any one of these on its own is a signal; together they are a near-certain identification. See how HomeInspecto logs each finding in a demo.
Why it worries buyers and insurers
The wiring itself was well made. The trouble is a century of age, modern electrical loads it was never built for, and what later owners did to it.
- 1No ground. Ungrounded circuits offer no safe path for a fault, so there's no protection for modern electronics or for a person touching a faulted appliance.
- 2Aged insulation. The original rubber and cloth turns brittle and cracks with decades of heat, leaving conductors exposed.
- 3Amateur splices. Decades of unqualified taps into the system leave loose connections that overheat and arc.
- 4Overloading. Circuits built for a few lamps now feed kitchens, window units, and power strips they were never sized for.
None of this means a fire is coming — it means the margin for error is gone. Track condition notes per circuit in HomeInspecto.
The one that matters most
Knob and tube buried in insulation
Knob and tube was designed to run through open air, which lets it shed heat. When later owners blow attic insulation over it, that heat has nowhere to go and the wiring can overheat — a leading cause of fires in older homes. Electrical codes generally prohibit covering active knob and tube with insulation unless a licensed electrician has inspected and certified it. If you find wiring packed under insulation, it is the single most important thing to flag.
The insurance reality
This is usually what forces the issue. Active knob and tube narrows a buyer's insurance options sharply.
- Many carriers will not write or renew a policy on a home with active knob and tube.
- Some accept it only once it is confirmed disconnected and abandoned in place.
- Others require a licensed electrician's written certification that it is sound and not buried in insulation.
- Where coverage is offered, options are limited and premiums run higher.
A clear report with photos is what lets a buyer shop carriers before the clock runs out. See the photo record in a demo.
Make the old-wiring call defensible
Photograph the knobs, the splices, and the buried runs, tie each to a location, and hand over a report a buyer can take straight to a carrier.
What rewiring costs
Buyers almost always want a number, and the honest answer is a wide range. A full rewire pulls new grounded cable through the house and abandons the old runs in place.
- PartialTargeting the riskiest circuits first — buried runs, damaged sections, kitchens and baths — costs less but may not satisfy a carrier that wants full removal.
- Full rewireWhole-home replacement runs well into five figures and varies widely with home size, wall access, and whether the panel also needs upgrading.
Because the spread is so wide, point buyers to a licensed electrician for a firm quote rather than a figure in the report. Attach the electrician referral to the record in HomeInspecto.
How to word it in a report
Knob and tube is a topic where wording matters. The goal is to inform without declaring a home unsafe or pricing a repair you aren't licensed to quote.
-
Do
- State that knob and tube is present, and whether it appears active or abandoned.
- Note condition — brittle insulation, visible splices, runs buried in insulation.
- Recommend evaluation by a licensed electrician.
- Photograph each finding with its location.
-
Don't
- Declare the home unsafe or a fire imminent from the wiring type alone.
- Quote a repair or rewiring price.
- Promise a carrier will or won't insure it.
- Call it a code violation without citing the specific condition.
Factual, photographed, and referred to an electrician is the wording that holds up. See the report templates in a demo.
How HomeInspecto handles old wiring
-
Condition notes per circuit
Record active vs abandoned, brittle insulation, and splices against each run you find.
-
Buried-run flagging
Mark wiring found under insulation as a priority item, with the photo attached.
-
Photo on every finding
Knobs, tubes, splices, and panels documented so the call is never your word alone.
-
Electrician-referral wording
Built-in language that refers the condition out without overstating the risk.
-
Carrier-ready export
A clean report a buyer can forward to insurers while shopping coverage.
-
Record per property
Every finding stays on file for the re-inspection after a rewire.
Honest to the buyer, defensible for you. Book a demo on your own workflow.
Frequently asked questions
Is knob and tube wiring always unsafe?
No. Undisturbed, in good condition, and not buried in insulation, it can still function. The risk comes from age, damage, amateur splices, and being covered by insulation. A licensed electrician should assess its condition — see how findings are documented in a quick demo.
Can you get insurance with knob and tube wiring?
Often with difficulty. Many carriers decline active knob and tube outright; some insure it only if it's abandoned or certified sound by an electrician, and coverage that is offered tends to cost more.
Why can't knob and tube be covered with insulation?
It was designed to run in open air so it can shed heat. Insulation traps that heat and can cause the wiring to overheat, a known fire risk, which is why codes generally prohibit covering active knob and tube unless an electrician has certified it. Flag any buried runs — start a free trial to track them.
How much does it cost to rewire knob and tube?
It varies widely with home size, access, and panel condition — a full rewire commonly runs well into five figures, while a targeted partial rewire costs less. Refer buyers to a licensed electrician for a firm quote.
How should an inspector report knob and tube?
State that it's present and whether it looks active, note its condition and any buried or spliced runs, photograph each finding, and recommend a licensed electrician — without declaring the home unsafe or quoting a price. See the wording in a demo.
Old wiring, clear report
Turn every knob-and-tube finding into a defensible record
Per-circuit condition notes, buried-run flags, photos on every finding, and electrician-referral wording — documented the way buyers and carriers trust.
