Polystyrene is a thermoplastic, so when heated melts and runs (and to some extent degrades back to its building blocks of styrene monomer, an HFL).
End result is that a small contents fire can get very rapidly out of control, as demonstrated by the near-endless list of food factories that have burnt uncontrollably.
Other factors involved are that the panels are mainly used internally, with simple butt joints, which have no mechanical integrity. Compare these with the complex joints used for external cladding panels.
Insurers are generally quite happy with polyisocyanurate panels approved under LPS1181. Much better performance! For convenience, we tend to regard these panels as being non-combustible (OK, not technically true, but this is only one of many issues our underwriters have to consider, and simplification is necessary).
But given large enough volume/fire load, the choice of panel becomes less significant, and sprinklers will be necessary.
Fire Risk assessments? Yet to see one which takes polystyrene into account, but I live in hope......... Some dreadful reports from various other countries of deaths related to the stuff.
And in relation to Ian Gough's comments above, I can but agree! The standard of so-called fire-stopping insurers see is generally dreadful: so we ask for 3PC to LPS 1132 whenever we get the chance.
Plenty of perfectly good uses for polystyrene, but composite panels are not on that list!
