If I rememebr right, Fiberfrax was a very heat resistant material but transmitted a lot of heat through it so it wasn't a particularly good insulator.  Maybe somebody else has some specifics.

What I'm going to be testing with a L -IV cowling where heat damage has ocurred is a material we make from particially carbonized PAN fiber (Polyacrylo-nitrile like the the PAN based structural carbon  but it's non-conductive and not very strong.  But it has great temperature resistance and insulating values.  I will be trying it in 1/8" and 1/4" thick blankets.  These will be covered with water proof aluminized high temp glass fabric.

I'll report the results when we are done.

Dan Newland