In any case, if you overheat Romex and destroy the insulation, you can create a line-to-ground-or-neutral arc or short. If you overheat knob and tube and destroy the insulation, you are quite unlikely to cause an actual problem — there’s nothing nearby for it to short to.
Modern insulation tolerates rather higher temperature than old insulation, and knob and tube has plenty of other problems, but this isn’t really one of them.
(If GCFIs on branch circuits or feeders were widely deployed, even at higher thresholds than 5mA, and everyone used either grounded metallic conduit or bundles of wire with a bare ground in the middle (like Romex or NM-B), then insulation failures would be quickly detected.)
Thanks. Good to know. But probably worth mentioning that, even though you can insulate around knob and tube where local jurisdictions allow it:
"K&T wiring is designed to dissipate heat into free air, and insulation will disturb this process. Insulation around K&T wires will cause heat to build up, and this creates a fire hazard. The 2008 National Electrical Code (NEC) requires that this wiring system not be covered by insulation."
I wonder how often this actually is a problem, in that the owner looks at knob and tube wiring and keeps it for the heritage value. My parents were ripping it out in the 1970s from a 1920s home. I know that heritage designations often require using the original construction techniques but do they prevent upgrades to the electrical system?
"I'm sorry sir but your grandparents didn't have grounded outlets so you can't install have them either."
The problem, at least in my case, is that my 1930 house has k&t wiring and little/no insulation. The easiest way to add insulation would be to blow it in through drilled holes. This is a lot less involved than rewiring the house.
I have no affection for the wiring - nostalgic or otherwise.
Presumably, you are entirely removing interior plaster&lathe/drywall to access wall cavity (and replace wires) rather than drill holes and blowing insulation?
If you are upgrading the insulation from the inside you'll have whatever is the inner wall out anyway, it's pointless otherwise; if you're doing it from the outside (possible with some roofs) it doesn't matter; and filling the wall cavity is done from the outside too, unless you have a really weird house no wiring goes there.
> If you are upgrading the insulation from the inside you'll have whatever is the inner wall out anyway
In Philadelphia in 1996, I had a company blow in insulation to my wood-framed, plaster+wood-lathe covered cavity walls without removing the inner walls. They created a series of 4"/10cm holes at the top of each bay and blew the insulation in that way.
ps. I later found out that this is a terrible idea for vapor control reasons.
Yeah, that's basically why just putting up new walls is the long term solution. And as a bonus you can put in some sweet CAT6 cabling and enough electrical sockets while you're at it. :)
Another benefit: when you take out the inner walls in an old house, you are going to find layers and layers of… stuff. Like 2cm of styrofoam (insulation in a nineties way) and a new layer of drywall on top of whatever chipboard/grout/newspapers where put in in a distant past (at least in my old house).
You don't have to remove all of the lathe and plaster. Blown in insulation after upgrading the electrical. Fish poles and long cuts repaired afterwards.
Most home owners would either prioritize replacing/upgrading knob and tube wiring because we like to have more outlets and circuits than when knob and tube wiring was installed, or because the building code or local permitting office would require the upgrade. Same thing for aluminum wiring (1970s?), GCFI, replacing fuses with breakers, etc.
