The Natural Philosopher wrote:

Get some and put a blowtorch to it.
Celotex insulation does not really burn.

Where "not really" means that an external flame will make it burn,
giving off a surprising amount of smoke, it will char and and the flame
from the foam will sputter out after a few seconds when you take the
blowtorch away ... maybe a big slab with lower surface:volume ratio
performs better than tests on offcuts of an inch or two?