Definition
This is old-school wiring that used ceramic knobs to hold individual wires in your walls. While it was fine back then, it's now a serious fire and shock hazard—and your insurance company probably won't touch it with a 10-foot pole.
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Knob-and-tube wiring presents serious safety hazards: insulation degrades over time, wires can overheat when in contact with insulation, and the system lacks grounding. Insurance companies often refuse coverage or charge premiums for homes with knob-and-tube wiring. Many jurisdictions require replacement before sale. Inspectors identify knob-and-tube wiring in attics, basements, and crawl spaces. Replacement with modern wiring is necessary for safety and insurability.
Pro Tip
If a home has knob-and-tube wiring, budget for complete replacement before closing. Most insurance companies require it, so this isn't optional—it's just a cost of entry.
Example
A home purchase in Massachusetts was delayed when the insurance company refused to provide a policy until knob-and-tube wiring was replaced. The homeowner hired an electrician to replace the entire system with modern Romex wiring, costing $6,000-8,000 but enabling the purchase to proceed.