Why is buried knob-and-tube wiring in contact with loose insulation a concern?

Knob-and-tube wiring was designed around air. The conductors are run separately, spaced apart and held clear of framing by porcelain knobs and tubes, and their rubber and cloth insulation was rated on the assumption that the heat generated in the conductor would dissipate into the open air around it. Bury that wiring in loose-fill or batt insulation and the heat has nowhere to go: the conductor runs hotter, the already-brittle insulation degrades faster, and the failure mode is a fire. This is why so many jurisdictions and insurers specifically prohibit covering knob-and-tube with thermal insulation, and why the finding is a safety item rather than a note about obsolete wiring. The inspector reports the buried wiring, notes that the extent cannot be determined without disturbing the insulation, and recommends evaluation by an electrician.

Knob-and-tube conductors are spaced in open air and rely on that air to dissipate heat. Covering them with insulation traps the heat, accelerating insulation breakdown and creating a fire risk.

Knob-and-tube wiring uses spaced single conductors relying on free air for heat dissipation. Covering it with thermal insulation is widely prohibited because trapped heat degrades the aged rubber and cloth insulation.

It Was Cooled by Air. Bury it in insulation and you have taken away the only cooling it ever had.

Look also for knob-and-tube extended with modern cable and buried splices, which is common and compounds the concern.

• •Reporting the wiring as merely obsolete rather than a safety concern
• •Estimating the buried extent from what is visible
• •Missing modern splices and extensions concealed in the insulation