Is it dangerous to use polyisocyanurate without covering it with drywall?
Polyisocyanurate, AKA: PIR, polyiso, or ISO –Wiki

During a fire, polyisocyanurate can release a considerably higher level of toxins than other insulating materials. Other than PVC (which will release dioxin), I can't think of a worse building material to inhale fumes from. Other types of foam might be 'better' but all of them should have a fire barrier. I haven't installed ISO in 20 years, others are much cheaper (like, half the price) and nearly the same R-value (~5 vs. 6).


  A 2011 study of fire toxicity of insulating materials at the University of Central Lancashire's Centre for Fire and Hazard Science studied PIR and other commonly used materials under more realistic and wide ranging conditions representative of a wider range of fire hazard, observing that most fire deaths resulted from toxic product inhalation. The study evaluated the degree to which toxic products were released, looking at toxicity, time-release profiles, and lethality of doses released, in a range of flaming, non-flaming, and poorly ventilated fires, and concluded that PIR generally released a considerably higher level of toxic products than the other insulating materials studied (PIR > PUR > EPS > PHF; glass and stone wools also studied).