What is Knob and Tube Wiring?

Knob and tube wiring was an early standardized method of electrical wiring in buildings, in common use in North America from about 1880 to the 1930s. It consisted of single insulated copper conductors run within wall or ceiling cavities, passing through joist and stud drill-holes via protective porcelain insulating tubes, and supported along their length on nailed-down porcelain knob insulators.

Elements
The ceramic knobs were cylindrical and generally nailed directly into the wall studs or floor joists. Most had a circular groove running around their circumference, although some were constructed in two pieces with "pass-through" grooves on each side of the nail in the middle.
By wrapping electrical wires around the knob, and securing them with tie wires, the knob securely and permanently anchored the wire. The knobs separated the wire from potentially combustible framework, facilitated changes in direction, and ensured that wires were not subject to excessive tension. Because the wires were suspended in air, they could dissipate heat well.
Ceramic tubes were inserted into holes bored in wall studs or floor joists, and the wires were directed through them. This kept the wires from coming into contact with the wood framing members and from being compressed by the wood as the house settled.

Capacity
Although these installations were adequate for the electrical loads at the time of installation, modern households use a range and intensity of electrical equipment unforeseen at the time. Homebuyers often find that existing K&T systems lack the ampacity needed for today's levels of power use. As household power use increased following the Second World War (because more appliances per household were being plugged in), first-generation wiring systems became susceptible to abuse by homeowners who would avoid repeatedly blowing fuses by overfusing the circuits, thus subjecting the wiring to heat damage due to higher levels of current.
Deterioration or abuse may have rendered it unsafe
Knob-and-tube wiring may also have been damaged by building renovations. Its rubber insulation will be dried-out, thus brittle when handled, or it may have been damaged by rodents or carelessness (for example, hanging objects off wiring running in accessible areas like basements).

Covering K&T with thermal insulation is unsafe and prohibitedsed). As a result, energy efficiency upgrades that involve insulating previously uninsulated walls usually also require replacement of the wiring in affected homes.

Insurance
As existing K&T wiring gets ever older, many insurance companies may deny coverage due to increased risk. Several companies will not write new homeowners policies at all unless all K&T wiring is replaced or an electrician or an approved home inspector has certified that the wiring is in good condition.