Can attic insulation cover old Toronto wiring?

No — attic insulation should never cover knob-and-tube wiring, and in Ontario it's actually prohibited by code. This is one of the most common and dangerous mistakes made during home insulation upgrades in older Toronto neighbourhoods, and it's a situation that requires an electrician's assessment before any insulation work proceeds.

Why Covering Knob-and-Tube Is Dangerous

Knob-and-tube (K&T) wiring was designed to dissipate heat into open air. The ceramic knobs hold the wire away from framing, and the ceramic tubes protect it where it passes through joists — the entire system depends on free air circulation to prevent the conductors from overheating. When you bury that wiring under blown-in cellulose or fibreglass batts, you eliminate the heat dissipation the system was engineered around. The insulation acts like a blanket, trapping heat generated by the wire under load. Over time — especially during Toronto winters when heating loads are high and circuits are working hard — this leads to insulation breakdown, arcing, and fire.

The Ontario Electrical Safety Code explicitly prohibits covering knob-and-tube wiring with thermal insulation unless a licensed electrician has inspected the system and confirmed it is in acceptable condition, properly fused, and not overloaded. Even then, many ESA inspectors and insurers take a harder line: if K&T is present and active, insulation stays out of that attic space until the wiring is replaced.

The GTA Housing Reality

This issue comes up constantly in Toronto's older housing stock — the Annex, Riverdale, Cabbagetown, High Park, Rosedale, Leslieville, and similar pre-war neighbourhoods where homes built between 1900 and 1945 still have original or partially original K&T wiring. Homeowners pursuing energy retrofits (a smart move given Toronto's brutal winters and high heating costs) often hire insulation contractors who either don't know about the wiring underneath or don't flag it as their problem. The insulation goes in, the wiring gets buried, and the fire risk increases significantly.

Making this worse: decades of amateur electrical additions are common in these homes. Someone in the 1970s may have spliced modern NMD90 cable onto the original K&T using wire nuts stuffed into the ceiling cavity — no junction box, no accessibility, no code compliance. Buried under 12 inches of blown cellulose, these connections become completely inaccessible and impossible to inspect.

Insurance is the other major issue. Most Ontario home insurers now require confirmation that K&T wiring is not present, or that it has been assessed and is not covered by insulation, as a condition of coverage. If you insulate over active K&T and later have a fire, your insurer has grounds to deny the claim. Some insurers are cancelling policies outright on homes with active K&T, regardless of insulation.

What You Should Do

Before any attic insulation work proceeds in a pre-1950 Toronto home, have a licensed electrician inspect the attic wiring. They'll assess whether K&T is present and active, check for improper splices, evaluate the condition of the cloth insulation on the conductors (it becomes brittle and cracks with age), and confirm whether the circuits are properly fused at the panel.

If K&T is found, your realistic options are: rewire the affected circuits before insulating (the right answer), or leave the attic uninsulated in areas where K&T is present (not ideal for energy efficiency, but safe). Partial rewires that address the attic circuits specifically can sometimes be done for $3,000–$6,000 depending on how many circuits run through the attic and how accessible the walls are. A full rewire of a typical pre-war Toronto bungalow runs $10,000–$15,000 and solves the problem permanently.

An ESA permit is required for any rewiring work. Once the circuits are replaced with modern NMD90 wiring and the inspection certificate is issued, your insulation contractor can proceed safely — and your insurer will have the documentation they need.

If you're ready to get a licensed electrician in to assess your attic wiring before insulation work begins, Toronto Electrical Repair can match you with local professionals at no cost. Browse electricians through the Toronto Construction Network directory at torontoconstructionnetwork.com/directory?trade=electrical.