There are several iterations of foam: rigid, plain styrofoam sheets, extruded, and expanded. The in the future two, expanded and extruded, are the most popular when it comes to home building projects probably, but what's the difference?
Expanded Often called Beadboard, sheets are created from stable beads of polystyrene. These beads happen to be expanded by injecting and dissolving a small amount of pentane gas into the base material during production. Under heating, the gas expands the beads to create closed cells. These cells are approx. 4o times the volume of the original bead, and can end up being molded to form insulation boards, blocks, or shapes. While a shut cell structure and resistant to water even now, it is not as resistant to wetness as XPS is going to be, but that may be splitting hairs. It really is generally cheaper than it 'extruded' cousin and scientific tests show EPS to get a lower result on the surroundings and can be recycled easier than XPS.
Extruded, this foam begins with sound polystyrene crystals. The crystals, along with other additives will be fed into an extruder. Within the extruder itself, ruthless and heat form the mixture into a plastic fluid. This thick liquid plastic is then forced into a die where it emerges and expands to a foam. This foam is shaped, cooled, and cut. Due to extrusion process, XPS has the capacity to maintain its thermal and/or physical properties even more uniformly than EPS and makes a great substitute for corrugated cardboard, thus it gained in level of popularity amongst architectural modelers. Another advantage of XPS is definitely it's strength: because it subjected to higher compression through the manufacturing process, it really is more rigid than EPS slightly. Also, as the closed cell structure is going to be finer than that of EPS indeed, XPS resists wetness better. I assume once this became the defacto amongst these modelers, it migrated over to railway modelers. Manufacturers often add dye, which has no influence on the XPS other than identification of the manufacturer.
So that is better? Well, after reading numerous content articles it does indeed seem that Extruded foam includes a few slight positive aspects after all. But i want to note the focus on slight. For most railway modelers with simple layouts, there probably is not any difference since the foam will be covered anyway by other materials such as plaster. In the end, it seems the three main differences are: Expense, where EPS gets the advantage; Durability, where XPS stands up a bit better and retains its rigidity, and finally, Moisture resistance, where once more, XPS includes a minor advantage.