Knob-and-Tube Wiring and Insulation in Ontario: Why Insulation Cannot Touch It, What the ESA and Your Insurer Say, What Replacement Costs, and Why the Wiring Comes Before the Attic Job

Insulation & Air Sealing

Knob-and-Tube: The Attic Cannot Be Insulated Until the Electrician Has Been, and There Is No Clever Way Around That

In an Ontario house built before about 1950, and some into the mid 1950s, the wiring in the attic may be two separate cloth-covered wires running on white ceramic knobs along the joists and through ceramic tubes where they cross them. That is knob-and-tube, and it was designed to be cooled by open air. Bury it in insulation and it runs hot, the rubber and cloth on the conductors, already seventy years old, bakes off, and the attic is a tinderbox with a wire in it. The Ontario Electrical Safety Code does not allow thermal insulation in contact with it, most insurers will not cover a house that has it live, and no insulation contractor worth the name will blow over it. This page is what that means for the attic job, in dollars and in order.

Ontario, checked October 2026. Electrical figures are from the electricians I work with and vary by house; insulation rates are the schedule the rest of this section uses, before HST. The ESA is the authority on the wiring, not this page.

How to recognize it

  1. In the attic: two single wires, not a cable, running 4 to 6 in. apart, each held off the wood on white or cream porcelain knobs and passing through porcelain tubes at every joist. Splices are soldered and taped, hanging in the open, with no box. If the attic has blown insulation already, the knobs poke up through it like mushrooms.
  2. In the basement: the same pairs along the joists, often beside newer cable where part of the house was rewired. A 60 amp fuse panel with knob-and-tube leaving it is the usual combination.
  3. At the outlets: two-prong receptacles with no ground, and switches that are a little warm. A house with some three-prong outlets may have been partly rewired, which is the common case; the attic is the last place anyone spends money and the first place the old wire stays.
  4. What it is not: old two-wire cable with a cloth jacket from the 1950s and 1960s is not knob-and-tube; it has its own problems but can be insulated over if the ESA is happy with it. If you are not sure, the ESA inspection below settles it.

Why insulation cannot touch it

A wire gives off heat in proportion to the current through it. Knob-and-tube was sized and spaced for that heat to leave by open air, which is why the two conductors run apart and off the wood. Put R-40 of cellulose over it and the heat has nowhere to go; the conductor temperature climbs, the seventy-year-old rubber on the wire, already cracking at every bend, cooks and falls away, and a bare copper wire sits in a bed of fibre. Cellulose is borate-treated and fibreglass does not burn, but the paper backing, the dust and the wood do.

That is the mechanism, and it is why the Ontario Electrical Safety Code does not permit thermal insulation in contact with knob-and-tube wiring. An ESA inspector who finds it buried writes it up as a defect to be corrected, which means the insulation comes back out at your expense and the wiring gets replaced anyway. The insurer side is simpler: most Ontario insurers will not write a new policy on a house with live knob-and-tube, and the ones that will want an ESA certificate and charge for the privilege. A buried run found on a claim inspection is the kind of thing that ends a policy.

Nothing on this page softens that. Insulation is never placed over or against knob-and-tube wiring. Not with a gap, not with a metal shield, not “just the one run to the light”.

What the ESA inspection and the rewire cost

The sequence is an electrician first, then the ESA, then the insulation. A licensed electrical contractor, and in Ontario only a licensed electrical contractor can take out the permit, tells you how much of the house is still live knob-and-tube and how much was replaced over the years. He files the notification with the ESA, does the work, and the ESA inspects and issues the certificate of acceptance, which is the piece of paper the insurer and the insulation contractor both want. The ESA permit and inspection run a few hundred dollars on a house rewire, on top of the electrician.

The rewire itself is the number people are afraid of, and fairly. On the 1,200 sq ft bungalows and storey-and-a-half houses I see in Simcoe County, electricians quote roughly $8,000 to $15,000 to replace the knob-and-tube, and a full two-storey with plaster walls and a finished basement can run $15,000 to $25,000 and up, with a new 100 or 200 amp panel on top if the fuse panel is still there. What moves it is access. An open attic and an open basement let the electrician fish most of the house from above and below and leave the plaster alone; a finished basement ceiling and a knee-wall attic mean holes in the walls and a drywaller after him. The attic job, which is why you called, happens to be the moment the attic is cleared.

Do the attic once

An attic with knob-and-tube and 3 in. of old fill has to be cleared for the electrician anyway; he cannot fish new cable through fifty-year-old rock wool and find the junctions. So the order is removal, rewire, ESA, air seal, insulate, and the removal is done once. Run it the other way, insulate first because the quote was good, and the electrician rakes the new R-60 aside to work and leaves it in heaps, or the ESA sends it out.

Worked on the 1,200 sq ft attic. Vacuum removal of the old dry fill at $2.00 is $2,400. Rewire of the attic circuits and the second floor by the electrician, say $9,000, with the ESA fees inside it. Then the attic job from bare joists: air sealing 1,200 at $1.00 is $1,200, which on a house this age includes the chimney dam and boxing the bathroom fan out through the roof; baffles $300; insulated hatch $200; blown cellulose to R-60 from bare at $3.75 is $4,500. Insulation side $8,600 before HST; everything including the electrician about $17,600. The removal page and the attic cost page carry the insulation side; the electrician gives you his own number.

Two things to say to the electrician while the attic is open. Replace every non-IC pot light with an IC-rated airtight fixture, because the insulation cannot touch those either; it is $45 a cover otherwise. And have him put the attic junction boxes where the air sealing can reach them, on the joist tops and not under the eaves where nobody can get at them after the blow.

The rebate and the knob-and-tube house

The attic rebate under Home Renovation Savings does not care about the wiring, but the participating contractor does: he will not blow over knob-and-tube and the rebate is paid on his work. So the rewire comes out of your pocket, and if the house qualifies, up to $1,250 of the attic work comes back afterward for an attic that was at R-12 or less and goes to R-50 or more, 50% of the after-tax cost up to the cap. Check the program site before you sign; the conditions include a house occupied more than six months and no previous attic rebate. The rebates page has the rest. There is no rebate for the electrical work from anyone.

If the whole house is being opened up, the older homes page is the one to read next, because a knob-and-tube house is also a house with no vapour barrier, empty walls and, often enough, vermiculite in the same attic, and the money has an order.

Questions people ask

Can you put insulation over knob-and-tube wiring in Ontario?

No. The Ontario Electrical Safety Code does not allow thermal insulation in contact with knob-and-tube wiring, because the wire was designed to be cooled by open air and overheats when buried. Reputable insulation contractors will not blow over it and the attic rebate will not pay for it. The wiring is replaced first.

Will my insurance cover a house with knob-and-tube?

Most Ontario insurers will not write a new policy on a house with live knob-and-tube, and those that will usually want an ESA certificate and charge more. Knob-and-tube found buried under insulation at a claim is the kind of finding that ends a policy. An ESA certificate of acceptance after a rewire is what the insurer wants to see.

How much does it cost to replace knob-and-tube in Ontario?

Electricians quote roughly $8,000 to $15,000 for a small bungalow with an open attic and basement, and $15,000 to $25,000 and up for a two-storey with plaster walls and a finished basement, plus a new panel if the fuse box is still there. Access drives the price; an open attic is the cheap case, which is why the attic job and the rewire are done together.

Does the ESA have to inspect a knob-and-tube replacement?

Yes. A licensed electrical contractor files the notification, does the work, and the Electrical Safety Authority inspects and issues a certificate of acceptance. That certificate is what the insurer asks for and what the insulation contractor wants before the blower starts.

Can I insulate the walls if the knob-and-tube is only in the attic?

Only the walls that have none in them, and on a house that old most outside walls carry a run to a switch or an outlet. Dense-packing a wall around live knob-and-tube is the same hazard as burying it in the attic. The electrician maps what is live before any wall is drilled and filled.

Price the attic that follows the electrician

The attic insulation cost calculator prices the attic from bare joists with the removal, air sealing, baffles, hatch, HST and the rebate check as their own lines, so you have the insulation number to put beside the electrician’s. If somebody has already quoted you to blow over the wiring, the $99 review will tell you what that quote did not ask.

Price my attic after the rewireReview my quotes, $99

Sources and further reading