Originally Posted by musigenesis
It's odd to think that there would be a need to insulate your insulation, but most likely (since the article also mentions ignition barriers) the thermal barrier in this case would be to protect the insulation for a short period of time from the high heat generated by a fire (so that it doesn't start burning and generating toxic gases).
In the case of a tiny bus, this isn't really important because you would already be crispy fried chicken by the time a thermal barrier would make any difference.
I understand what you're saying. And in the likely majority of situations where heat or direct exposure to spark or flames could cause the foam to catch, I imagine the situation may be as you describe. But I can also envision other circumstances where an otherwise controllable fire, or spot heat source, could ignite and/or cause the foam to gas off, turning something slow & controllable into something fast and not in a quick hurry. This is conjecture - I admit having a limited understanding of foam technology or the variables associated with it - but to my way of thinking that's all the more reason to consider adhering to code standards.
If nothing else, it could help make the job less hazardous for fire fighting peeps forced to deal with it, and/or others in the vicinity of your really bad day.