Knob-and-tube wiring and insulation
The NEC bars concealed knob-and-tube wiring in hollow spaces filled with insulation that surrounds the wires. For housing regulated by HCD, California allows insulating around it only after a California-licensed electrical contractor surveys and certifies the wiring and other conditions are met.
What the code requires
Concealed knob-and-tube wiring may not be used in commercial garages, theaters and similar locations, motion-picture studios, or hazardous (classified) locations. It also may not be used in hollow wall, ceiling or attic spaces insulated with loose, rolled or foamed-in-place insulation that surrounds the conductors.
Under 394.10, new knob-and-tube wiring is allowed only to extend an existing installation, or elsewhere by special permission.
California's 2025 code adds an exception for housing regulated by HCD. Insulation may go in around knob-and-tube wiring only if a California-licensed electrical contractor surveys the wiring and certifies it is in good condition, with no deterioration, improper overcurrent protection, or improper splices or connections. The certification is filed with the building department, and the owner gets a copy.
The insulation contractor must post caution notices in English and Spanish at all accessible insulated areas. The insulation must be noncombustible as defined in the California Building Code and contain no electrically conductive material. Repairs, alterations or extensions to the wiring need permits and inspections, and the building department may also require permits for the insulation.
On the job
- Attic insulation added during an energy retrofit without the electrical contractor's survey and a filed certificate is a common problem.
- Insulation that does not qualify as noncombustible under the California Building Code does not meet the California exception.
- Repairs or extensions to knob-and-tube circuits done without permits are another frequent issue.
Exceptions that matter
- California only, for HCD-regulated housing: insulation around knob-and-tube is allowed when all six conditions of the state exception are met, as described above.
In California
The insulation exception is a California amendment, not part of the national NEC. It requires a survey and certification by a California-licensed electrical contractor, filed with the building department, before insulation is installed around knob-and-tube wiring.
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