We have a rental house that still has some knob and tube wiring. We did not know about the knob and tube wiring when we bought the house 18 years ago. We didn't know such a thing existed but remember the seller telling us her father was an electrician and updated some of the wiring. Some of the knob and tube has been removed but there is still some in the house. Last year we hired an electrician to remove it but because it is an old house he wasn't sure if he could get all of it out. It ended up he installed AFCI instead and said we should be safe. There is some lose insulation in the attic over the wire. Should we hire someone else to remove the knob and tube wire? Is the AFCI enough of a safety feature that we should not have to remove the knob and tube? thanks for your help!
I maintain 100 year old electrical systems.
Knob & tube is legit. Notice that most of the people telling you it's bad are people who stand to profit from your worry.
Cities and the NFPA are not nearly as worried.
AFCI is a superb way to protect any wiring you're worried about. I recommend it for aluminum wiring, backstab connections, you name it. It will prevent arcing/sparking of the sort which makes connections hot and starts fires, not that K&T has a problem with that. Like most electrical work of that age, K&T splices are wildly overbuilt by modern standards: wrapped, soldered and insulated often with two kinds of tape. We use backstabs.
People of that age didn't have our tech, but they were serious about their craft. Nobody had to tell them not to eat Tide pods.
Add GFCI protection to stop even small current leakage from wet wires, and you are safe as can be. You can even put 3-prong sockets on those receptacles.
One more thing. There was a concern that blown-in insulation packing around knob and tube wire would cause it to overheat. However, the states of Oregon and Washington did an extensive survey on this, and found it to be false. As such, they liberalized their laws to permit blown insulation around knob and tube.
Well, look at it this way, knob and tube wiring is a wiring method that is almost 100 years old. It was most probably installed when the house was originally built. The house has not burned down yet. Hence, it must be a pretty sound wiring method.
If the loose insulation were to get wet between conductors it could begin to heat up. If your roof is sound and the attic is dry, then you are good.
Normally, the only reason to update wiring is because you are doing a major remodel and opening up the walls anyway.
The AFCI breakers add a layer of protection to the older system. I wouldn't worry about it.
Mixed K&T and new is pretty much the rule, not the exception. It's just too hard to change the switch legs to light fixtures, and there's essentially no point doing so. See https://diy.stackexchange.com/a/20279/5960
Some insurance companies will insure if "90%" of the K&T is gone. It's unclear if that's 90% of the wire length, 90% of the circuits, 90% of the load or what. It's probably unclear because they have no clue!
Early AFCI devices had troubles on "shared neutral" systems like your K&T. Recent ones, pretty much have that figured out.
AFCI devices on K&T circuits protect against the same types of issues they do on modern circuits. The K&T is likely fine itself, but a spark could develop on an outlet box edge or if the wire is physically damaged.
So AFCI on K&T: a good idea.