So, here's what happens to foam core board at 100 deg C for an

So, here’s what happens to foam core board at 100 deg C for an hour, with light pressure applied.

Next test: Will it still collapse if it’s not under pressure?
That will have to wait for another day. It’s way past my bedtime.

maybe you should try mineral wool…

or heat resistant silicone…
http://www.smooth-on.com/a59/Mold-Max%3D-60-Higher-Heat-Resistant-Silicone/article_info.html

I know there are better options for insulation. I just wanted to see if something I already had would work.

I wouldn’t go there. Just find a place where they are building houses. Ask them for some glass-wool or stone-wool. That is the best you can get. It doesn’t have to be renewed either.

Be carefull!! You want to use thick rubber gloves to handle this stuff. It contains microscopic glass fibers that stick in your skin and break off. You’ll get itchy hands and it feels like hundreds of tiny splinters in your hands. (DO NOT RUB YOUR EYES WHILE HANDELING THIS!!!)