The answer
Knob-and-Tube (pre-1940s): ungrounded, cloth insulation, fire risk when covered by thermal insulation. Single-strand Aluminum (1965-1975): thermal expansion loosening, oxidation, galvanic corrosion (CPSC: 55x more likely to reach fire-hazard conditions; fix with copper pigtailing/AlumiConn/COPALUM). Romex NM-B (modern): PVC-sheathed copper hot, neutral, and bare ground rated 90°C.
Sources
This answer is drawn from the standards and guidance below. It is general information about how these systems work — not a diagnosis of your property.