According to Steve Barker :
Well at least it doesn't burn like fiberglass, and support combustion in a
big way like the foam does.

Except for the binders, fiberglass can't burn - silica isn't
combustable, and the amount of binder is relatively small.
Whereas, depending on the cellulose formulation, virtually _all_
of it will burn sooner or later.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.