I have a Beemax hive and it is definitely a very sturdy, dense, "commercial grade" of polystyrene - unlike anything I have seen in stores or at the home improvement places.
I tried to use cut sheets of the cheapest polystyrene that I could buy at Home Depot to insulate my wood hives on the inside for winter. The bees almost immediately started to pick it apart and chew it out. They apparently didn't want it in there at all. So I just removed it. (I was experimenting to see if sliding cut sheets of that ridged insulation inside the hive would work for the winter warmth, but it wasn't to be.) They don't pick apart or chew at the Beemax hive at all.