Derating of Romex cable starts at the rating of the cable: 60 degrees Centigrade.
This reduced rating vs the underlying THWN-2/ or THHN reflects the fact that the cable sheath insulates...
The rise in super-insulated homes may well cause the Code Committee to revisit this issue. But as it stands, no NEC provision exists to derate based upon spray in foam. For my money it should be treated as if were running underground.
