A drain hole is a small opening moulded into the base of a polystyrene box, letting meltwater run out. Some boxes have one, some have several, and some have none at all. Which you want depends entirely on what you're packing.

When you want them

Wet fish, shellfish and other ice-packed goods produce meltwater as the ice does its job, and that water has to go somewhere. Without a drain hole it pools in the bottom of the box, and the stock ends up sitting in it. Fishmongers and seafood wholesalers packing loose ice around fresh fish generally want a box with drain holes, so the water can run off during transit or once the box is set down at the other end.

When you don't

Pharmacies, clinical labs and bakers using gel ice packs have the opposite need. A gel pack holds its melt inside a sealed pouch, so there's no water to drain, and a hole in the base becomes a way for outside air, insects or moisture to get in. For anything where the box needs to stay sealed against the outside, a solid base without drain holes is the better fit.

Checking before you buy

Drain-hole presence is one of the fields worth checking on any product listing alongside internal size and wall thickness, since it isn't always obvious from a photo. If a supplier hasn't published whether a box has drain holes, that should show as not stated.

• Packing wet fish or shellfish on loose ice: look for a box with drain holes
• Packing gel ice packs, dry ice, or anything that must stay sealed: look for a box without drain holes
• Not sure which your supplier's box has: check the listing, or follow through to the supplier's own page to confirm

Gel ice packs and dry ice do the same job in different ways, and the difference changes what you need from the box as much as what you put inside it.

How gel ice packs work

A gel ice pack is a sealed pouch of water and a gelling agent, frozen solid before it goes into the box. As it warms back up it absorbs heat from its surroundings while it changes from solid to liquid, and it keeps doing that steadily until it has fully thawed. Because the process is gradual, gel packs are suited to holding a chilled temperature, around 0°C to 5°C.

Wall thickness is what determines how long that cold lasts, because a thicker EPS (expanded polystyrene) wall slows the rate at which outside heat reaches the pack and the goods around it. Beyond that, a gel pack asks nothing unusual of the box. It sits inside a standard lidded container and needs no ventilation of its own.

How dry ice works

Dry ice is frozen carbon dioxide, and it does not melt the way gel packs do. It sublimates, meaning it passes straight from solid to gas, and that gas has to go somewhere. It runs cold enough to keep goods frozen, but it changes what the box has to do.

A box holding dry ice cannot be sealed completely airtight. As the CO2 gas builds up inside a fully sealed container it has nowhere to go, and pressure increases. The lid needs to allow that gas to escape, whether through a loose-fitting lid type or purpose-made vent holes, which is a different requirement from the drain holes used for meltwater.

Dry ice is also cold enough to burn unprotected skin on contact, so it is handled with gloves. Couriers generally treat it as a dangerous good for transport with its own rules, separate from the parcel size limits that apply to the box itself, so it is worth checking with the courier directly before dry ice goes anywhere by post or courier network.

Matching the method to the box

• Gel ice packs: a standard sealed box, wall thickness chosen for the hold time needed, no ventilation required.
• Dry ice: a box with a vented or loose-fitting lid, gloves for handling, and confirmation of the courier's rules before sending.

Neither method is better in general. Which one fits depends on whether the goods need to stay chilled or stay frozen, and the box has to be set up for the one actually being used.