A neighbor of ours got an energy audit last winter, and the recommendation was the obvious one for a house of that age: blow insulation into the attic, because there was almost none up there. Then the insulation contractor put his head through the hatch, saw white porcelain knobs on the joists with bare-looking wire strung between them, and told him the job was off until an electrician had been in. My neighbor's reaction was the one most people have — the wiring had worked fine for ninety years, and the insulation was going on the floor, not on the wires. Both of those are true. Neither is the point.
Recognizing it from the attic hatch
Knob-and-tube is unmistakable once you've seen a photograph. Two separate conductors, each running on its own, held off the wood by white ceramic knobs that look like small spools nailed to the joists. Where a wire passes through a framing member it goes through a ceramic tube set into the drilled hole. There is no outer jacket, because there is no cable — the hot and the neutral travel apart, sometimes on opposite sides of a joist bay, and meet again only at a device.
The insulation on the conductors is usually rubber under a woven cloth braid, often varnished and long since gone dark and brittle. If your house was wired between roughly the 1880s and the 1940s and nobody has gutted it since, some of this may still be live behind the plaster even where the visible parts of the system look modern.
The design assumption everything else follows from
The spacing isn't decorative. This system was engineered around air: conductors run in free space with clearance from each other and from anything combustible, and the surrounding air carries away the heat that current produces. That is the cooling method. There is no conduit and no assumption the wire will ever be enclosed, and that single design choice explains nearly every rule written about this wiring since.
It also explains why age alone isn't the problem. A circuit that was installed well, never overloaded, and still sitting in open air is doing what it was built to do. Hazards come from changing the conditions it was designed around, and burying it in insulation is the largest change available.
The Code line that turns a weatherization job into a violation
Article 394 of the National Electrical Code covers concealed knob-and-tube wiring, and its Uses Not Permitted section is short. The wiring is not allowed in commercial garages, in theaters and similar locations, in motion picture studios, in hazardous classified locations, and — the one that reaches ordinary houses — in hollow spaces of walls, ceilings, and attics where those spaces are insulated by loose, rolled, or foamed-in-place material that envelops the conductors.
The reasoning follows from the design assumption. Blown-in, rolled, or foamed insulation stops heat from dissipating into the free air the conductors were relying on. Conductor temperature climbs, brittle rubber degrades faster, and a system whose margins were built around open air is operating without them. The Code isn't calling the wiring defective — it's saying you can't take away the thing keeping it cool.
Worth being precise about what this means for an existing house: the Code governs new work, so an installation that was legal when it went in isn't retroactively outlawed. What it does govern is anything you do from here — additions, alterations, extensions, and the insulation job.
Two things the system never had
Beyond heat, knob-and-tube is missing pieces modern circuits take for granted, and no amount of good condition supplies them. There is no equipment grounding conductor. The system was two wires, hot and neutral, which is why the outlets fed by it have two slots. Anything expecting a ground — surge protection, many appliances, the safety path that carries a fault back to the breaker instead of into a person — has nothing to connect to.
The second gap is that the conductor insulation was never meant to last a century. Rubber embrittles with age and heat, and where it has been disturbed by generations of attic work, or baked near a chimney or a fixture, it crumbles off the copper and leaves bare conductor. That is why a system can be sound in one part of a house and dangerous in another, and why an assessment means an inspection rather than a verdict about the house's age.
How one state built a legal path through it
Washington State's rules are unusually explicit about this, and reading them tells you what a serious assessment involves. Under WAC 296-46B-394, loose or rolled thermal insulation may go into spaces containing existing knob-and-tube only after an appropriately licensed electrical contractor has surveyed the wiring and certified in writing to the state that it is in good condition — no improper overcurrent protection, no insulation failure or deterioration, no improper connections or splices. Repairs, alterations, and extensions have to be inspected.
The rule then constrains materials and protection. The insulation must meet Class I specifications with a flame spread factor of 25 or less, and foam may not be used with knob-and-tube at all. Every circuit has to be protected per the NEC ampacity table's 60-degree-Celsius column, using circuit breakers or Type S fuses — the tamper-resistant type that can't be swapped for an oversized one.
Most states do not offer that pathway, and your own jurisdiction may prohibit covering it outright or say nothing at all, which usually means the base Code rule applies. The value in reading Washington's version is that it spells out the three questions any competent electrician works through anywhere: is the wiring sound, is every circuit correctly protected, and has anyone been splicing into it.
What a homeowner can check without touching anything
All of these are observations, not tasks. None require opening a device or handling a conductor:
- Two-slot outlets through much of the house, especially if a few were swapped to three-slot without rewiring — a three-prong receptacle on an ungrounded circuit is a false promise.
- Porcelain knobs and tubes visible from the attic hatch or in an unfinished basement, with conductors running separately rather than in a jacketed cable.
- Existing insulation already lying over or around those conductors, which is the situation the Code rule is written about.
- A fuse box rather than a breaker panel, especially with fuses larger than 15 amps on circuits feeding old wiring.
- Splices where old cloth-covered wire meets modern cable, particularly any made outside a junction box.
- Anything scorched, crumbling, sagging, or taped, and any conductor with bare copper showing.
- Circuits that dim or trip when a space heater or window unit runs, a sign the old wiring is carrying loads nobody imagined for it.
If the last two appear, the conversation stops being about insulation and becomes about safety on its own terms.
Questions worth asking before anyone prices the work
Ask for an assessment rather than a quote, because quotes only mean something once somebody knows what's up there. Ask whether the electrician will document the wiring's condition in writing, and whether they'll map which circuits are still knob-and-tube and which were replaced — most old houses are a patchwork, and the ratio drives the cost. Ask what protection each remaining circuit has and what it should have, and whether the plan splices new cable onto old conductors.
If replacement is the answer, ask what gets opened up and what gets fished, since that drives the price and the plaster repair both. And ask your building department what your jurisdiction requires before insulation goes over any of it.
Living with it in the meantime
Full replacement is a real project and rarely happens the week you learn about it. The sensible interim is to stop making the circuits work harder: keep space heaters, window units, and other heavy loads off the old wiring, skip the power strips on circuits sized for a couple of lamps, and leave the two-slot outlets alone rather than fitting three-prong receptacles that look grounded and aren't. Answer your insurer honestly if they ask. And hold the insulation project behind the electrical one — the order matters more than the schedule.