Why does foamed polystyrene insulate hot drinks better than unfoamed polystyrene?

Foamed polystyrene insulates hot drinks more effectively than solid (unfoamed) polystyrene because of its cellular structure. During the manufacturing process, a blowing agent is incorporated into the polystyrene resin, which releases gas to create a spongelike mass filled with tiny, trapped bubbles.12

This structure, often referred to as a closed-cell structure, significantly enhances the material's thermal insulation properties. While solid polystyrene is a dense, rigid plastic, the foamed version traps air within its cellular matrix. Because air is a poor conductor of heat, these trapped pockets act as a barrier that slows the transfer of thermal energy from the hot liquid to the outside environment, thereby keeping the drink hot for a longer period.32