TIL from an old master electrician about knob and tube wiring failures

I was working with a 30 year veteran last week on a rewire in an old house downtown. He pointed to some knob and tube wiring and said most people think it fails from age, but it's actually the insulation getting brittle from heat cycles. He showed me how the real danger is when people add extra insulation in the attic, trapping heat around the wires. That made me rethink how I explain hazards to homeowners. Have any of you seen fires actually start from knob and tube, or is it mostly just the insulation breakdown causing shorts?

2 comments

2 Comments

nancy_owens25d ago

That's EXACTLY the thing right there people shove loose fill insulation right on top of those wires and then wonder why they have electrical problems later. The heat has nowhere to go so the insulation cooks and cracks way faster than if it had some airflow.

brooke53325d ago

Actually the bigger issue is when people use the wrong type of insulation near recessed lighting or knob and tube wiring. Loose fill isn't automatically a problem if it's the right kind and installed with proper clearance. I've seen attics where they packed loose fill cellulose tight around IC rated cans and it was fine for decades. The real nightmare is when someone puts non-IC rated fixtures under a blanket of loose fill and then wonders why the thermal cutoff keeps tripping. That's where the heat buildup actually causes the wire insulation to degrade fast, not from just being covered in fiberglass.