EZSMART Corporation, ESA/ECRA #7012690 , North York , Ontario
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Topping up attic insulation is one of the most common DIY and contractor projects in Toronto and GTA homes, and it almost always means burying existing electrical cable under a foot or more of batts or blown-in fill. Homeowners reasonably worry: is that a fire risk? The answer splits cleanly between modern cable and one specific legacy wiring type. This guide covers exactly where the line is.
Modern NMD90 cable is rated for direct contact with thermal insulation and is routinely buried under attic fill without issue — the 90°C insulation rating on the conductors accounts for the reduced heat dissipation. The one major exception in Ontario homes is knob-and-tube wiring, which was designed around open-air cooling and is explicitly prohibited from being covered by insulation under the Ontario Electrical Safety Code. If your attic has knob-and-tube and blown-in insulation on top of it, that is a defect requiring correction.
Knob-and-tube wiring runs conductors through open air, spaced apart on ceramic knobs and through ceramic tubes, relying on that air gap to dissipate heat. Its rubber insulation was designed for that open-air environment, not for being buried. Cover it with insulation and heat has nowhere to go — the conductor runs hotter than its insulation was ever rated for, accelerating the exact rubber degradation that leads to cracking and exposed copper. It is one of the most common (and most invisible) fire hazards we find during Ontario attic inspections, because the insulation itself hides the problem from view.
See what is knob and tube wiring and should I replace it and can old knob and tube wiring cause a fire.
This is why blown-in insulation contractors do not stop to route around modern cable — it is a non-issue for wiring installed in the last several decades.
Aluminum branch circuit cable from the 1965–1975 era is not prohibited from insulation contact the way knob-and-tube is, but aluminum’s tendency to loosen at terminations over time means any circuit under review for insulation coverage is also worth having checked for AlumiConn or CO/ALR remediation at its devices. See is aluminum wiring dangerous in a house.
Common discovery during a resale inspection or insurance renewal in the GTA. The fix is either removing the insulation from directly over the knob-and-tube runs, or (more commonly and more permanently) replacing the knob-and-tube circuit itself so the insulation question becomes moot. A licensed contractor can advise which is more practical for your attic layout.
The single most common insulation-related call we get in the GTA is a homeowner who topped up attic insulation themselves, then years later during a sale discovers knob-and-tube buried underneath — invisible until an inspector goes looking. If you are about to insulate an older Ontario attic and are not 100% sure what wiring is up there, spend the fifteen minutes to have a licensed contractor do a quick visual pass before the insulation crew arrives. It is far cheaper to check first than to remove insulation later to find out what is under it.
Yes, when the circuit was properly sized for the derating. Standard practice in Ontario homes.
It was designed to cool in open air. Insulation traps heat it was never rated to handle, accelerating insulation breakdown.
A visual look for ceramic knobs and tubes is safe. Confirming the full circuit condition should be a licensed contractor.
Yes. Only IC-rated fixtures can be directly covered; non-IC fixtures need clearance maintained.
Insulated knob-and-tube is a common flag on Ontario insurance inspections and can affect coverage.
Planning an attic insulation top-up in a Toronto or GTA home and want the wiring checked first? Call ezsmart at 416-838-9006 or use our contact page. An ESA-licensed electrician will identify your attic wiring before the insulation goes down.
Electrician Since 2008 Journeyman Electrician Designated Master Electrician at EZSMART Corp
