Heat tolerance of Fibreglass

Resource for this? not faced, just raw fibreglass.

Unfaced fiberglass Melts at around 1100 degrees and I don't know what temp it can catch fire.
Chimney Fire is 2000+ degrees, so decent chance fiberglass may melt during a chimney fire if in contact with liner.
Mineral Wool insulation (same technique as fiberglass but different material) melts at over 2100 degrees and will maintain integrity during a chimney fire.

Lastly, neither stops air flow. If one wants something to stop air flow, use a piece of sheet metal.

A chimney fire can reach over 2000 degrees inside the liner, the outside during one will be cooler just don't know what the outside would be.