Insulation over knob and tube wiring

• Put nothing over it. No batts, no loose fill, no boxes of Christmas decorations resting on a run. Insulation is the thing this wiring was designed not to have.
• Do not pull existing insulation off it either. If material is already lying over the wiring, leave it and call an electrician. Dragging a rake through old cloth covered wire is how a live conductor gets exposed.
• Do not touch the wires to test them. The covering on old runs goes brittle. It can look intact and come off in your hand.
• Do not judge live or dead yourself. Switching a breaker off does not prove which wire in the attic went quiet. That is a job with a tester and a plan, not a guess.
• Stop for anything burnt. Scorch marks, a melted look, a smell of hot plastic or a warm spot on a ceiling means an electrician today, not a quote next month.
• Watch the footing. This is still an attic. Step on the framing, not on the ceiling between the joists, and keep a hand free.

One more, and it is the reason this page has no quote form on it. The honest answer here is often "call an electrician", and this site does not connect anyone with electricians. Find one yourself, the same way you would for any other electrical work, and ask whether they work on knob and tube before they drive out. The pattern matches the vermiculite page, which carries no form either, for the same reason: the trade you need is not the trade this site connects people with.

You can do this from the hatch with a flashlight. You are looking for two giveaways above everything else. Ceramic knobs nailed to the sides or tops of the joists and rafters, with wire looped around them and tied off. And ceramic tubes set into the holes where a wire passes through a piece of framing. Both are white or brown, both look like plumbing parts more than electrical parts, and neither has any modern equivalent.

The third giveaway is how the wires travel. Knob and tube runs as two separate single wires with air between them, often a hand-width apart. Modern house wiring runs as one cable with everything inside it. Once you have seen that split, you cannot unsee it.

Two traps in that table. A house can have both kinds side by side, so finding modern cable proves nothing about the wire two joists over. And the wire is only part of the run. The knobs and tubes stay behind long after somebody has cut the wire out, so bare porcelain on a joist is a sign the house had this system, not proof it still runs on it.

While you are up there with a light, the same look answers other questions. What the insulation is and what else the attic is telling you both start from the same hatch. If any of it looks like loose gray-brown pebbles, stop on that instead and read the vermiculite page first.

This is the sentence that matters most on the page. An abandoned run and a working run look identical. Same knobs, same tubes, same cloth covering, same dust. Nothing about the appearance carries the answer.

It stays confusing because of how houses get updated. Rewiring usually happens a room at a time, or a floor at a time, or during one remodel that never got finished. The new cable goes in and the old wire gets left where it lies, because pulling it out means opening walls. So a single attic often holds some runs that were retired years ago and some that still feed a bedroom light.

Three homeowner tests that feel like proof and are not:

• Turning off a breaker or pulling a fuse. That tells you a circuit went dead. It does not tell you which of the wires overhead belonged to it.
• A pen style voltage tester. A reading of nothing can mean the run is dead. It can also mean the light switch feeding it is off, or that the tester could not read through old cloth. Nothing is not the same as safe.
• The wire looks ancient. Age says nothing about whether it is energized. Plenty of live knob and tube looks exactly as bad as the dead kind.

What settles it is an electrician who traces the runs and tests them under load. Ask for the result in writing, run by run, saying what is live and what is dead. That letter is the document the insulation contractor needs, and it is also what a buyer's inspector will want later.

No price appears in this table. This site holds no measured price for rewiring, for an electrician's hour, or for an evaluation, so it publishes none. Ask two electricians and you will have a real local range, which is better than a national number anyway.

A fourth route gets suggested online and it is worth naming so you can rule it out. Boxing or tenting a live run so the insulation goes around it is not a homeowner project. Federal attic guidance allows for wiring that has been "replaced or properly boxed", but proper is the load bearing word, and it belongs to the electrician and your building department. Improvised cardboard, buckets and rolled batts are the failures the rules exist to stop. That mistake shows up around recessed light fixtures too, for the same reason.

Find the row that matches what you saw. Every row ends in an action.

The recommendation. Get the wiring evaluated first, then insulate, and do not let the order flip because a crew has a free Tuesday. If the runs come back dead, you have lost a week and gained a letter that helps you sell the house. If they come back live, replacing those circuits is the route that ends the problem, and it is the one to price first. Leaving strips of ceiling bare is a delay rather than a fix: you pay most of the cost of an attic job and get part of the result. Whichever way it goes, book the wiring work and the insulation work in that order, because the second one buries the first.

Once the wiring is settled, nothing here is unusual anymore. The order of work takes it from there, air sealing comes before depth, and the R-value page carries the target for your climate zone.

The mechanism is the whole story, and it explains why this wiring is treated differently from the rest of your electrical system.

Every wire gives off heat when current runs through it. Modern cable is built and rated to run buried in framing and packed in insulation. Knob and tube was built the other way around. The knobs hold it off the wood, the tubes carry it through the wood without touching, and the two conductors run apart from each other in open air. Air moving around the wire is not a side effect of that design. It is the design. Pack insulation around it and the heat has nowhere to go.

The U.S. Consumer Product Safety Commission put the general version of this in writing on February 6, 1980. It told homeowners to have their electrical systems checked by a qualified individual before they install thermal insulation. Its words: the fire potential from damaged wiring or over-fused circuitry is "greatly increased when the heat given off by the wiring (which normally is dissipated through free air circulation) is confined by thermal insulation". Over-fused means the fuse or breaker is too big to protect the wire behind it.

The same release put numbers on it. More than 2,000 household fires a year happened when thermal insulation was ignited by faulty wiring or lighting fixtures, which the agency put at about 3 percent of an estimated 75,000 electrical system fires a year in American homes. Two honest limits on that. Those counts are from 1980 and this page has not found a current federal update. And the release is about old and faulty wiring in general. It does not name knob and tube.

The electrical code carries the specific version. The National Electrical Code, published by the National Fire Protection Association as NFPA 70, is the model code the trade works from, and it has restricted insulation in contact with concealed knob and tube wiring for decades. Treat that as background rather than as a citation. This page prints no code section number and quotes no code text, because neither was read for it. Ask your electrician or your building department for the wording that governs your house.

The National Electrical Code is a model code. Your state or city adopts an edition of it and can change what it says, so what applies to your house is a local question. Your building department and a licensed electrician are the authorities on it.

The clearance question follows the same rule. Some weatherization programs set a distance that insulation has to be held back from live runs. This page prints no figure for it, because none was read from its own standard here, and because the number that governs your attic is the one your electrician and your building department give you.

Usually stop, and that is the good outcome.

The federal guidance the trade works from is direct about it. The attic checklist from the U.S. Department of Energy and Pacific Northwest National Laboratory, in their Building America Solution Center tells the crew this. "If there is active knob and tube wiring present in the attic, insulation shall not be installed until wiring is replaced or properly boxed." Its guides for blown insulation and for batts start the job the same way. "Before any retrofit work is done, inspect the work area for active knob and tube wiring, bathroom fans venting into attics, roof leaks, and other hazardous conditions."

So what you should expect, from a crew that is doing this properly:

1. They go into the attic before quoting. Not just to the hatch. Knob and tube in one corner is easy to miss from the opening.
2. They name it and stop. The scope goes on hold rather than getting adjusted around the wiring.
3. They ask you for an electrician's sign off before they rebook. Some will not put a crew back in the attic without it in writing.
4. They requote afterward. The job may be cheaper than the first number if the electrician removed a problem, or dearer if the attic has to be worked around.

The crew to worry about is the one that says it is fine and blows anyway. You will not be able to tell from the paperwork. The depth measures correctly, the invoice reads R-49, and the wiring is under it where nobody will look again for twenty years. That is the same shape of failure as a blown attic with no baffles at the eaves: the invoice is right and the attic is not.

One more reason a contractor stops. Some will not warranty work over wiring they were told to ignore, and program funded work usually cannot proceed at all. Being handed your attic back is not a brush off. It is the answer.

This catches people out at the worst moment, usually mid purchase. Some carriers price a house with active knob and tube differently. Some ask for it to be removed from service within a set time. Some will not write the policy at all. Others do not ask.

This site has not surveyed insurers, so it publishes no figure, no share and no company name. What it can give you is the order to ask in, because the answer changes what the wiring work is worth.

1. Call your own agent before you spend anything. Ask what your carrier does with active knob and tube, and whether your current policy was written knowing it is there.
2. Ask what proof they accept. An electrician's written evaluation, a permit and inspection record, or an invoice for the rewire. Find out which one closes it.
3. Ask what happens at renewal, not just today. A policy that renews for years is worth more than a quote that lapses.
4. Get it in writing. A phone call is not a policy term, and the person who answers may not be the person who underwrites it.

This changes the math on the three routes above. If your carrier is going to insist on a rewire anyway, then spending money to insulate around live wiring buys you nothing and delays the thing you have to do regardless. If you are selling within a year or two, the same logic applies: the buyer's lender and the buyer's insurer will find it.

Two trades, two documents, one order. Keep them separate and keep them both.

• From the electrician. Which runs are live and which are dead, run by run. What was disconnected and where. Whether anything is left in the attic that must not be covered. And a plain statement that the attic can be insulated, if that is what they found.
• From the insulation contractor. Confirmation that they walked the attic and saw the electrician's letter. The area being insulated, in square feet. Any area deliberately left bare, and why. The target depth in inches and the target R-value.
• From your building department. Whether the electrical work needs a permit and an inspection where you live. It usually does, and the record is worth having when you sell.
• For the attic itself. Ask that anything left uncovered gets marked, with a note or a photograph. The next person up there will be adding depth and cannot see what is buried.

Doing it yourself is the wrong instinct on this one, and it is the only page in this section where that is flatly true. Reading the attic is yours. Testing wires is not. Do it yourself or hire works through where that line normally sits on attic work.

We do not sell insulation and we do not install it, which is why this section can tell you to stop spending. An attic with live knob and tube is not an insulation job yet. It is an electrical job with an insulation job behind it. Booking the second one first wastes the visit, and a careful crew will hand the attic straight back to you.

Three more cases where the answer is to do less than you were planning.

• You have knobs and tubes but no wire on them. The porcelain outlives the system by decades. If an electrician confirms the wire is gone or dead, the attic is ordinary and this page is finished with you.
• You were about to rewire the house to fix a cold bedroom. Those are two separate problems. Get the wiring evaluated because it is wiring, and price the comfort problem on its own. The signs page covers what a cold room actually points at.
• Your attic is already close to the level ENERGY STAR recommends for your climate zone. Within about R-10 of it, more depth pays back slowly. That threshold is this site's own estimate rather than a measured figure. The depth checker turns a tape measurement into an R-value range. Sort the wiring out anyway, because that is a safety question and not a payback question, and then leave the depth alone.

What is never optional: live knob and tube under insulation. That one gets cleared whatever else you decide about depth, air sealing or comfort.

• Recessed lights, IC ratings and clearance
• The hatch, scuttle or pull-down stair
• Bath fans that vent into the attic
• Inspecting an attic from the hatch
• Whether opening walls is worth it, and drilling and filling them, where old wiring is hidden
• Knee walls and rooms over a garage, which often hold old runs
• Vermiculite and asbestos, the other page here with no form on it
• The order attic work has to happen in