To all the concern of toxic smoke. Yes this is a rigid rule in commercial buildings and codes tend to not enforce the 'combustibles in a plenum' on single family residential rules.
I believe icynene will burn when exposed to flame, but it does not sustain a flame itself.
Many places you can line wooden floor joists for a return duct, the electrican has romex running through the joists, there will be plastic pipe in there.
So I would hope he made it out of the house before his roof caught on fire.
The other way of looking at it is you draw a central return from a living room with a synthetic carpet. The extension chord for the Christmas tree lights catches the carpet on fire and and 55 toxic gases billowing from the carpet. This all gets sucked into the central return.
Are there many jurisidctions out there where they are forcing you to install duct mounted smoke detectors in homes to provide fan shut down?