Does a polystyrene box leak? Material and joints are not the same thing

An intact EPS body takes up almost no water; leaks come through poorly fused beads, the lid line, corners and the base. Test the box with water while it is empty on receipt.

22 March 2026Updated 28 September 2026

Short answer

An intact polystyrene body does not let water through: EPS is about 98 percent air in closed cells and takes up almost no water even when immersed. Leaks come through poorly fused beads, the gap between lid and body, broken corners and punctured bases. Where there is meltwater or product liquid, use a closed inner bag or an absorbent pad.

An intact polystyrene body does not let water through. EPS is about 98 percent air by volume, its cells are closed, and it takes up almost no water even after long immersion.

Water gets out between the beads and at the joints rather than through the material itself. The EUMEPS EPS White Book notes that water absorption depends heavily on how well the beads are fused; in a poorly fused box, water finds its way through the gaps between them. A gap between lid and body, a broken corner, a punctured base or a drain hole does the same.

On receipt, set the empty box on a level floor without its outer carton, pour a little water into the base and watch the outside, especially the corners, for wetness. Where meltwater or product liquid is expected, use a closed inner bag or an absorbent pad, and never patch a food box with tape.