Not sure, but 334.80 doesn’t seem to differentiate between types of thermal insulation.
It just states “where multiple type NM cables are installed in contact with thermal insulation without maintaining spacing between cables, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15[B][2][a].”
I think the “spacing between cables” could be the troublesome issue.
From what I can gather, the concern over the spray-foam insulations may have been what started this revision, but when NEMA found no significant difference with the use of NM cables embedded in different types of insulation materials, it looks like they just decided to apply it generically across the board.
I can see yet another possible installation issue with regard to insulated ceilings. Electricians who just like to lay all their NM cable runs on top of furring strips [strapping] in ceilings of new residential construction, will have to somehow maintain wire spacing. I guess now they might just as well staple the NM to the sides of the ceiling joists and be done with it.
