Knob and tube wiring in Ontario

What knob and tube is

Knob and tube was the standard way to wire a house from roughly the 1880s to the 1940s, which covers a great deal of Toronto, Hamilton, and every older Ontario town. Individual conductors run through porcelain tubes where they pass through joists and are held off the framing by porcelain knobs, so the wires sit in open air.

That was a sound design for its era. Air is excellent insulation, and the system was built with generous clearances. Two things have happened since: the wiring is now 80 to 100 years old, and people have filled the cavities around it with insulation.

How to spot it

Look in the basement, in the attic, and anywhere joists are exposed:

• White or black ceramic knobs, roughly the size of a spool, nailed to joists with a wire wrapped around them
• Ceramic tubes where wires pass through drilled framing
• Two separate conductors running parallel with a gap between them, rather than one cable with everything inside
• Rubberised or cloth sheathing, often brittle or crumbling at the ends
• Two-prong outlets throughout the house, with no ground pin

Two-prong outlets are the giveaway you can check in ten seconds. They mean no ground, and no ground usually means original wiring somewhere.

Is it actually dangerous?

This deserves a straight answer rather than a scare. Knob and tube is not inherently unsafe, and intact, undisturbed, correctly loaded knob and tube has run for a century without incident in a lot of houses. The real problems are what has happened to it since:

• No ground conductor. Modern appliances and anything with a three-prong plug expect one. This is the fundamental limitation and it cannot be retrofitted onto the existing wire.
• Buried in insulation. The system relies on air circulation to shed heat. When someone blew cellulose into the walls or rolled batts over it in the attic, that cooling disappeared. This is the single most common real hazard, and it was created by a well-meant energy upgrade.
• A century of amateur modifications. Junctions taped rather than boxed, extensions spliced in, circuits extended to feed loads nobody imagined in 1925.
• Brittle insulation. The rubber sheathing hardens and cracks with age, particularly near heat.
• Overloading. These circuits were designed for a lamp and a radio, not a kitchen full of appliances.

The insurance problem

In practice, insurance is what forces the issue rather than safety. Most Ontario insurers will either decline to write a policy on a home with active knob and tube, or require an electrician’s certification that it has been removed or de-energised, usually within a set window after purchase.

This matters most at closing. Buyers of century homes routinely discover during the insurance binding process, days before closing, that the house is uninsurable as wired. If you are buying an older Ontario home, ask about the wiring before you waive conditions, not after.

How much does rewiring cost in Ontario?

The variable that moves the number is not really the size of the house, it is access. Fishing new cable through open stud bays is straightforward. Fishing it through a century home with plaster and lath, diagonal bracing, and balloon framing is slow, and every wire pulled means plaster repaired afterwards.

Which is why the timing advice is simple: rewire while the walls are already open. If you are gutting a kitchen, doing a whole-home renovation, or opening up walls anyway, the marginal cost of rewiring is a fraction of doing it as a standalone project. Done on its own it is mostly a plaster repair job with some electrical in it.

Two related notes. First, a panel upgrade to 200A often accompanies a rewire, both because the old panel is usually 60A or 100A and because anything you plan next, a heat pump, an EV charger, solar, needs the headroom. Second, all electrical work in Ontario must be done under an ESA permit and inspected. That is not optional, and the certificate is what your insurer will want to see.

For the wider picture on older homes, see our century home renovation guide.

Zeybek Construction

General contractors, Ontario

Written by the Zeybek team from more than two decades of renovation and construction work across Ontario, from Toronto and the GTA to Kitchener, Waterloo, and beyond. We build kitchens, bathrooms, whole homes, additions, and commercial spaces, and we handle permits and inspections as part of the job.

Related guides

Older homes

What does it cost to renovate a century home in Ontario?

Energy

How much does insulation cost in Ontario?

Whole homes

How much does a whole-home renovation cost in Ontario?

Last updated July 16, 2026. This guide is general information for Ontario homeowners and is not a substitute for a site-specific quote or municipal advice. Costs, rebate programs, and rules vary by municipality and change over time.