Knob-and-tube wiring in the attic: what it is and California's insulation rules

Knob-and-tube is the wiring many older Bay Area houses were built with: single conductors strung between porcelain knobs and threaded through porcelain tubes, running separately through the attic and walls. It was designed to shed heat into open air. That's the whole reason insulation and knob-and-tube have a complicated relationship, and why California's electrical code has a specific rule for it.

This page covers how to recognize it without touching it, what the code says word for word, how Berkeley turns that into a form, what it means for the Title 24 energy rules, and the order the work has to happen in.

What knob-and-tube wiring looks like from the hatch

You don't need to climb in, and you shouldn't reach for anything. With a flashlight from the top of the ladder, look along the tops of the ceiling joists and the rafters:

• Porcelain knobs, usually white, fastened to the side or top of the framing, with a single wire wrapped or tied to each.
• Porcelain tubes where a wire passes through a joist or stud.
• Two wires running apart, often several inches from each other, rather than bundled together in one flat plastic or cloth-jacketed cable.
• Fabric or rubbery wire covering, often darkened, sometimes brittle.

Seeing knobs doesn't tell you whether the wiring is still in use. Some houses were rewired decades ago and the old runs were simply abandoned in place; others still feed lights and outlets through it. You can't tell which by looking, so treat every wire as live, and don't move insulation around it to get a better view. Take a photo and leave the rest to an electrician.

If you find a mix, with newer sheathed cable alongside knobs, that's common too. It usually means part of the house was rewired and part wasn't.

What the California Electrical Code says

The rule lives in section 394.12 of the California Electrical Code. Berkeley's Form 146, revised in March 2026 and citing the 2025 edition, prints it in full. Concealed knob-and-tube may not be used in "hollow spaces of walls, ceilings, and attics where such spaces are insulated by loose, rolled, or foamed-in-place insulating material that envelops the conductors."

Then comes the exception. Insulation isn't prohibited where knob-and-tube is present, provided all six of these are met:

1. A survey and certification. The wiring is surveyed by an electrical contractor licensed by the State of California, who certifies that it is in good condition, with no evidence of deterioration or improper over-current protection, and no improper connections or splices. Any repairs, alterations or extensions need permits and inspections.
2. Filing. The certification is filed with the building department, and the property owner gets a copy.
3. Notices. Every accessible area where insulation has been installed around knob-and-tube is posted by the insulation contractor with a clearly visible notice that caution is required when entering. The notice is printed in English and Spanish.
4. Noncombustible insulation, as the California Building Code defines it.
5. Nothing conductive. The insulation can't have electrically conductive material as part of it or supporting it.
6. Local permits still apply. The building department can still require permits and inspections for the insulation itself.

Item 4 is the one that narrows the choice of material. Whether a particular product meets the Building Code's definition of noncombustible is something to confirm from its documentation and with your building department, before it's ordered, not after it's in. For how the common attic materials behave around heat and flame in general, see whether attic insulation is flammable.

Berkeley's Form 146

Berkeley is one of the clearest examples in the Bay Area, because it puts the rule on a single page. The city says plainly that attics with knob-and-tube wiring may not be insulated unless a certification letter from an electrical contractor is submitted. Form 146, the Knob-and-Tube Wiring Safety Report, is that letter.

The certifying electrician fills in their company, state license number, city business license and license class, and ticks one of three boxes:

• The existing knob-and-tube wiring is in good condition, with no evidence of deterioration, proper over-current protection, and no improper connections or splices.
• The wiring is not in good condition and may not be covered without repair or rewiring.
• The wiring is disconnected and not live, or has been removed.

Copies of the completed form are posted in the attic before inspection, and the original goes to the building inspector at final inspection. Warning signs in English and Spanish go up at every attic access where knob-and-tube is concealed by insulation. The second page of the form is the sign itself: "WARNING: Knob and Tube Wiring. Use Caution When Entering Attic", with the Spanish text beneath it.

Other cities handle the paperwork their own way. Oakland, for example, runs insulation-only work through an insulation certificate. Wherever you are, the electrical code's conditions are statewide; the form is local.

The Title 24 exemption almost nobody mentions

Adding attic insulation normally brings in the 2025 Energy Code's altered-ceiling upgrades (the rules), and live knob-and-tube that stays is one of the exemptions.

The Energy Commission's 2025 compliance manual lists a few situations that are exempt from all of those upgrades. One of them is an attic with knob-and-tube wiring that is not being removed.

What that means in practice:

• If the knob-and-tube stays, the energy code doesn't require you to reach R-49 or do the other altered-ceiling items. But if you insulate around it, the electrical code's six conditions above still apply in full.
• If the knob-and-tube is removed or the house is rewired, the exemption no longer applies, and the altered-ceiling rules for your zone come back into play when insulation goes in.

Neither path is wrong. It's a decision about the wiring, made with an electrician, that happens to decide which insulation rules you're under.

The order of work

Getting the sequence right avoids paying twice. Insulation laid down before the electrician has been through often has to come back up.

1. Spot it. From the hatch, or at an inspection. If we find it, we stop and refer you to an electrician; Attic WISE doesn't do electrical work.
2. Electrician's survey. A California electrical contractor, the C-10 classification, surveys the wiring and gives a written finding: good condition, needs repair or rewiring, or disconnected or removed.
3. Repairs or rewiring, if needed, under their own permit and inspection.
4. Certificate filed with the building department, copy to you. In Berkeley, that's Form 146.
5. Insulation, chosen to meet the noncombustible and non-conductive conditions if the wiring stays live, or to meet the altered-ceiling rules for your zone if it's been removed.
6. Notices posted at each attic access in English and Spanish, where insulation now covers live knob-and-tube.
7. Building department inspection, with the form posted in the attic.

If you're planning a rewire anyway, it's usually simplest to finish it before the attic is insulated, while the electrician can still see and reach everything.

Walls are a separate question

Knob-and-tube often runs through the walls as well as the attic, and the same code section covers wall cavities. Filling closed walls around live knob-and-tube needs the same survey and conditions. The wall insulation page covers how closed walls get insulated once that's settled.

Next steps

If you think you've seen knob-and-tube, take a photo from the hatch and leave it be. Then talk to an electrician before anyone quotes insulation. When their part is done, book a free inspection and bring their written finding; it decides which materials and which code rules apply. For other loose-fill questions, see blown-in insulation, and if the wiring is one of several old-house questions, insulating older Bay Area homes takes them era by era.