Knob-and-Tube Wiring: Old, Ungrounded, and Often Uninsurable

Knob-and-tube (K&T) wiring was standard in US homes until roughly the 1940s: single insulated conductors strung through ceramic knobs and tubes. Undisturbed, it can work for a very long time. The trouble is that most K&T is no longer undisturbed.

The actual risks

• No ground conductor. Three-prong outlets on K&T circuits are often ungrounded, which is a shock hazard.
• Brittle insulation. Cloth and rubber insulation degrades with age and heat.
• Buried in insulation. K&T was designed to shed heat into open air. Blown-in attic insulation packed around it changes that.
• Amateur splices. Decades of DIY tie-ins to newer wiring are where fires actually start.
• Overloaded circuits. It was sized for a few light bulbs, not a modern kitchen.

How to tell from the listing and the showing

• Build year before about 1950 with no mention of an electrical update
• Two-prong outlets throughout, or three-prong outlets that a $10 tester flags as ungrounded
• A small or fuse-based panel in the photos
• Ceramic knobs visible in unfinished attic or basement joists
• Listing phrases like "original charm throughout" or "partially updated electrical"

"Partially updated" is the important one. It usually means live K&T remains somewhere.

Insurance, lending, and cost

Many insurers decline or surcharge active K&T, and some require an electrician's letter. Full rewiring of an occupied house commonly runs well into five figures, depending on size, plaster walls, and access. Get an electrician to estimate how much K&T is still live. That number belongs in your offer, not in your first year of ownership.