On 18/06/2017 11:35, The Natural Philosopher wrote:

Get some and put a blowtorch to it.
Celotex insulation does not really burn.
I set some on fire quite easily with the sparks of a metal grinding disk
when using an off-cut of Cellotex as a spark deflecter (thinking the
stuff would be fireproof). The sparks were hitting the non-foiled end
grain and I'm sure it actually had flames coming from it not to mention
lots of quite nasty smoke.