Short answer
Knob-and-tube is old wiring in older homes. Undisturbed it can be safe, but damaged insulation or past modifications create fire risk — have it inspected.
What knob-and-tube wiring is
Knob-and-tube was the standard residential wiring method from roughly the 1880s through the 1940s: single insulated copper conductors run through porcelain tubes where they pass through wood framing and supported by porcelain knobs, with the hot and neutral wires kept separated in open air. Millions of older homes were wired this way, and a surprising amount of it is still in service.
In its original form it was a reasonable system for its era — but it was designed for a world of a few light bulbs per room, not for modern electrical loads. And crucially, it has no ground wire, which matters for the safety of today's appliances and electronics.
When it becomes dangerous
Knob-and-tube in pristine, untouched condition is less alarming than its reputation suggests — the problems come from what the decades did to it. The original rubber-and-cloth insulation becomes brittle and crumbles with age, especially in hot attics. Well-meaning modifications over the years — splices buried without junction boxes, connections to modern wiring done improperly, circuits extended beyond their original design — are where most of the real danger lives.
Blown-in attic insulation is a particular concern: knob-and-tube was designed to shed heat into open air, and burying it in insulation can cause overheating. If your older home has had insulation added over old wiring, that combination deserves an electrician's assessment.
- Brittle, crumbling insulation from age and attic heat.
- Improper splices and modifications made over the decades.
- Buried in attic insulation it was never designed for.
- No ground wire — unsafe for modern three-prong appliances.
- Circuits extended far beyond their original light-only design.
Signs your old wiring needs attention
You may never see the wiring itself — it hides in walls and attics. The warning signs show up in how the house behaves: frequent blown fuses or tripped breakers on old circuits, outlets or switches that feel warm, flickering lights, a burning smell with no obvious source, or two-prong outlets throughout the house (a hallmark of ungrounded wiring).
If you are buying an older home, a general home inspection may flag knob-and-tube, but a licensed electrician's focused evaluation tells you its actual condition — whether it is intact and serviceable, or damaged and due for replacement.
What an electrician does about it
An electrician inspects the accessible wiring — attics, basements, crawl spaces — checking insulation condition, splices, junction boxes, and how the old system interfaces with any newer wiring. They can also test grounding and identify which circuits lack it.
The outcome is not automatically 'rewire the whole house tomorrow.' Options range from targeted repairs of damaged sections to phased replacement of the most stressed circuits, up to a full rewire for systems that are broadly deteriorated. An honest electrician explains what they found, what is urgent versus what can be planned, and what each option involves — get that assessment in writing.
Insurance and resale realities
Some insurers are reluctant to cover homes with active knob-and-tube wiring, or they charge more for it — ask your insurer directly rather than discovering it at claim time. On resale, buyers' inspectors will flag it, so having a professional assessment (and any completed remediation documented) smooths the process.
If you smell burning, see scorching around old fixtures, or find crumbling insulation, do not wait for a convenient time — have a licensed electrician evaluate it promptly.