Is Knob-and-Tube Wiring in an Old Hailey or Bellevue House Dangerous?

How to tell what you have

Knob-and-tube is easy to recognize once you see it. Two separate wires, one hot and one neutral, run a few inches apart through the framing. Where they pass through a joist or stud, they go through a porcelain tube. Where they run along the wood, they are held off it by porcelain knobs. Look in the attic, the basement or the crawl space, where nothing covers the framing.

Later houses and later additions often have the next generation of wiring: cable with a cloth or rubber jacket around two conductors, often with no ground wire. Plenty of older valley houses have both, plus modern cable wherever someone remodeled.

Two-prong outlets are a clue but not proof. A house can have modern wiring and old outlets, or knob-and-tube behind three-prong outlets someone swapped in. What is behind the cover plate is what counts.

Why old wiring is common here

The valley has had electric service longer than most of Idaho. The Advisory Council on Historic Preservation's profile of Hailey notes that the town had the first telephone and electric light service in the territory. The older neighborhoods in Hailey and Bellevue have houses that are close to a century old or more, and many have been added onto more than once.

That history is why an old house here can hold three or four generations of wiring at once. The original runs in the oldest part, cloth-jacketed cable from a mid-century addition, and modern cable in the kitchen. Nobody has a drawing of where one stops and the next begins.

What actually makes it dangerous

The wrong assumption runs both ways. Some owners hear knob-and-tube and assume the house is about to burn down. Others reason that it has worked for a hundred years and will keep working. Neither is a plan. The wire's age matters less than what has been done to it.

Knob-and-tube was designed to shed heat into open air, with the two conductors spaced apart. Bury it in insulation and that heat has nowhere to go. Hang a space heater and an extension cord off a circuit that was laid out for a few light bulbs and it is carrying loads it was never meant for. Our post on why breakers trip more in winter covers that part.

Then there are the splices. Every owner who added an outlet, a ceiling fan or a light over the last century had to connect to something. Some of those connections were done well. Some were twisted together, taped and left in a wall or an attic, and the original insulation on the wire is brittle with age where it has been handled. An inspection is mostly about finding those.

Do not insulate over it

This is the one that matters most in a valley house, because the first thing many owners of an old, cold house want to do is blow insulation into the attic and the walls.

The National Electrical Code's article on concealed knob-and-tube wiring does not allow it in hollow wall, ceiling and attic spaces that are insulated with loose, rolled or foamed-in-place insulation that envelops the conductors. That rule exists because the system depends on air around the wire.

So the order of work is wiring first, insulation second. Have an electrician find out which runs are still live. Live runs that sit where insulation will go get replaced or rerouted first, and disconnected runs can be confirmed dead. If insulation has already been blown over live knob-and-tube, have it looked at before winter loads go on it.

Repair, partial replacement or a full rewire

There is no single answer, and anyone who gives one without opening the attic is guessing. The code limits concealed knob-and-tube to extensions of existing installations, so adding to it is not how anyone modernizes a house. The choice is between leaving sound sections in place for now and replacing them.

A full rewire makes the most sense when walls and ceilings are already open for a remodel, when insulation is going in, or when an inspection turns up bad splices and damaged insulation throughout. Replacing the circuits you can reach from the attic, basement and crawl space, and leaving the rest for a later remodel, is a reasonable middle path in some houses.

Rewiring is permitted work with an inspection. Our guide to which jobs need a permit covers how that works with the state, and our guide to doing your own electrical work covers what Idaho allows a homeowner to do in their own house.

A three-prong outlet on a two-wire circuit is not a grounded outlet, whatever it looks like. If you need grounded outlets for a computer or an appliance, ask an electrician what the code allows on an ungrounded circuit before swapping anything.

Buying an old house in Hailey or Bellevue

If the listing says updated electrical, ask what was updated. A new panel does not mean new wiring past it. A general home inspector can flag knob-and-tube they can see, but they are not going to trace circuits through finished walls.

Ask your insurance agent before closing whether the wiring affects your policy, and get that answer in writing. Then have an electrician look at the attic, the crawl space and a few outlets before you sign. That gives you something to plan around, and it is far easier to rewire an empty house than one you have moved into.

Our post on what drives the price of electrical work explains why a job in an older house is hard to price before the covers come off.