Greg, I couldn’t swear that it was NMC, but years ago I worked in several homes that were wired with some form of white-jacketed NM cable that had TW conductor insulation and plastic filler strands instead of paper. These homes were probably wired sometime in the mid to late 70’s or possible the very early 80’s.
I agree that the claims of wicking or capillary action of moisture through the paper filler is BS at best under most all circumstances.
I’m pretty sure we’ve all seen it happen before, but it was always due to some installation issue other than just having the NM stapled above a gazebo or covered porch, crawlspace or a similar, arguably damp location. That is, at least in the situations where I have encountered it.
