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Hello. Welcome to Just Answer.
I have been doing electrical work full time for over 20 years, and I've crawled through many attics that had knob and tube wiring.
I would estimate that more than 95% of those attics had blown in insulation as well.
It's very rare to find the pink bats of insulation anymore.
from when I was a kid I guess :)
so what do you think?
I just don't see a reason to replace the insulation, as this is the first time I've ever heard anyone suggest this or mention it as a risk.
does knob and tube wiring have some hidden risk if it's old that i don't know about?
Knob and tube wiring has inherent risks, due to a few factors. I would be happy to list them for you.
also, insulation laying on top of knob and tube is ok?
that would be great
I've never heard that it isn't. I've had to uncover many splices on knob and tube wiring that were buried under blown insulation. It's just expected now when I crawl into the attic of an older home to troubleshoot a problem.
One risk of knob and tube is that it only has one layer of insulation over the copper conductor, wheres Romex type wire has two layers.
Another risk is that the splices are never in junction boxes, so any arcing from loose connections would not be contained to a box.
so what would you recommend as the best course of action. do nothing? let it be and call someone is there's a problem?
Yes, that's how I would handle it.
You're likely getting a better R-value from the blown insulation anyway.
and what is an R-value?
That is a number that correlates to the insulating capacity of the insulation... its ability to keep air that is at the desired temperature from leaving the living space, and air that is at an undesired temperature from entering the living space.