I've been cutting down a steel door to fit a short old basement doorway and am currently cutting out the foam core at the bottom so that the interior bottom rail of the door can be replaced. The foam core looks like standard styrofoam, but I noticed that it seems to be fiber-reinforced. I've lived in an old house for long enough to assume that all building materials are deadly until proven otherwise, so I'd like to know what these fibers are. Just fiberglass? Something even more benign?
It's a newly bought Jeld-Wen steel clad exterior door.