Why does condensation form on a polystyrene box?

Condensation does not mean the box is leaking. Moist air meeting a cold surface can leave water inside or outside, and where the water appears tells the sources apart.

27 March 2026Updated 28 September 2026

Short answer

When moist air touches a cold surface of the box and cools below its dew point it leaves water, the same event as droplets on the outside of a cold drink glass. Where the water first shows separates the sources: the outer surface and lid edge are usually outside condensation, while water on the inner base is product moisture, entering air or melting ice. Liquid with colour or odour points to a leak from the product, not condensation.

Moist air that touches a cold surface and cools below its dew point leaves its water there, even when the box is intact. It shows most often on the move from a warm store to a cold loading area, and the wet floor creates a slip risk.

Where the water is	Likely source	How to tell
Outer surface	Ambient air condensing on a cold surface	Wipe it, open the lid; if the inside is dry it is outside condensation
Lid edge	Outside condensation running down the edge	Check beneath the lid edge
Inner base	Product moisture, entering air, loading humidity, melting ice	Look at the colour and smell of the water

Write down the surface where it first appeared, loading and ambient conditions and the number of lid openings. Liquid with colour or odour is not condensation; separate the product and box and quarantine them.