What is an alternative to fiberglass?

Fiberglas is the correct spelling from the German usage. It is or was a registered trademark of Owens-Corning Illinois, and was so-stated in a NYC fairgrounds guide-book in l965 as a ( structural material of the future) The British prefer the term glass Fibre. Plexiglas ( again one S) is a specialized type for aircraft Windows and windshields, extra-durable.