Rather than piss about with a blow lamp can I suggest that you look at Saint Gobain's website and read thye statement that states, following Grenfell and their own admission of a flaw in their test results, that supplies of Celotex are suspended.
Play all you like with blow lamps but as somebody who has been involved in fire resistant testing at professional test houses I can tell you that you have no idea of the effects of air movement which feeds oxygen to the flame, and sustained flame application. Remember that the Grenfell panels once exposed to flame did not have the opportunity to "self extinguish", the flame continued to be applied to the foam, with the results that we all saw.
Me? I would not put this kind of insulation on my garage walls unless I knw=ew that it wold not burn and at this time nobody can give that assurance.