Bei expandiertem Polystyrol oder EPS handelt es sich um einen 'closed-cell' thermischen Kunststoff, der in Gestalt einer Polystyrolkugel Formern zugeführt wird. www.styreneforum.org/glossary_index_de.html
I haven't found a German equivalent to closed cell, but a few cases where the English was used.

Closed cell refers to the way the material is formed. Most likely, in this case, to keep it from soaking up water (or worse)
See: Foam plastics can be synthesized in an "open cell" form, in which the foam bubbles are interconnected, as in an absorbent sponge, and "closed cell", in which all the bubbles are distinct, like tiny balloons, as in gas-filled foam insulation and floatation devices.
http://encyclopedia.fablis.com/index.php/Plastic

Vielleicht kennt jemand einen deutschen Begriff dafür??