ᐅ Quality Differences in Expanded Polystyrene (EPS) Boards for Thermal Insulation?

Hi, I’m new to the forum and hoping to get some helpful answers. I want to insulate the exterior of my old house (built in 1929) using polystyrene insulation boards. To save costs, I plan to buy the insulation boards myself and provide them to my contractor. I’ve found very affordable polystyrene boards online that cost only about half of what my contractor charges. I’m curious if there might be quality differences—are the cheaper boards any good? I thought all polystyrene boards were basically the same and that you only need to check the thermal conductivity rating (WLG), or am I mistaken? What do the experts think?

To be honest, I doubt that the tradesperson would install it that way or for a lower cost. What would they gain from it, since they can probably source it cheaper themselves.
Besides, Styrofoam is a brand name—you probably mean EPS (expanded polystyrene). There are differences, especially regarding the surface as a base for plaster—the components need to be compatible.
Also, consider how thick it needs to be. I haven’t heard of any cases where this was ever checked, but if it comes back wrong, you might have to remove everything again.

1) Are both products made of polystyrene foam (brand name!) or perhaps something else?
2) Would the contractor even use materials sourced from outside?
3) Who is liable for product defects?
4) Who is liable for installation defects?
5) What happens if parties from points 3 and 4 blame each other?

It has already been agreed with the contractor that I will purchase the polystyrene boards myself; the company will take care of everything else. I was recommended boards that are 14 cm (5.5 inches) thick. The boards I plan to buy are called "WDVS - exterior insulation with Neopor 031/032" and are offered on eBay. If desired, I can provide the link here.

In case of product defects, the seller is responsible, which is the same situation when a plasterer brings his own boards, as he also obtains them from a supplier.

Neopor is not Styrofoam. But it is an external thermal insulation composite system (ETICS). ;-)

However, that does not answer questions 3 to 5. In case of doubt, you end up caught in between.

How much do you save by doing that?
If it’s only about 1000€ (around $1100), I would really recommend not doing it.
As mentioned before, if the plaster falls off again because the contractor used the wrong mortar for your tiles, then the trouble begins.