the first one (old one) was just expanding foam but the new one (bottom picture) is carved polystyrene. Its worth it in the end. Try it you will be surprised at how creative you can get when doing these!
Yeah, is there a health difference or does the polystyrene react differently to like the sealants and pants etc? I just watched some videos where after making the foam structure they cover it with cement grout anyways. Only asking because my craft store might not sell polystyrene... If its easier to work with thats one thing but if I'm covering it anyways I could use whatever foam I figure will keep shape. They added plastic skulls and whatever they wanted to the structure knowing they would just cover everything with grout and paint over it anyways.
I use 3/4" Insulating Foam Board from home depot. It is very dense, adds insulation to your walls (easier to heat the viv, more heat stays in) and is very strong. I then seal it with Minwax Sanding Sealer for Hardwood Floors. I scatter sand in the final coat to add texture and get rid of the shine.