Top, Bottom or Wrapped: A Four-Pack Placement Test in a 20-Litre Box

Four identical 20-litre EPS boxes, four identical 1 kg gel packs each, four arrangements, and one 30 degree room for 36 hours. The payload was 8 kg of water bottles at 4 degrees, with a logger in the centre and another on the lid. All four packs on the bottom held the payload under 8 degrees for 21 hours. Two on the bottom and two on top held it for 29. Nothing was added and nothing was spent.

Placement is the free variable in cold-chain packaging and the one most often decided by habit. This is what the four arrangements actually did, why the ranking is not what most packers expect, and how to write a placement diagram that a temporary worker will follow at 2 a.m.

The Four Arrangements and What Each Delivered

All packs were conditioned at minus 20 for 48 hours and loaded within four minutes of each other. Arrangement A put all four on the base. B put two on the base and two on top of the payload. C put one on each of the four side walls, standing upright. D wrapped two packs in a layer of bubble wrap and placed them beside the payload rather than against the wall.

Time to reach 8 degrees at the payload centre: A, 21 hours. B, 29 hours. C, 24 hours. D, 26 hours. Time for the lid logger to exceed 8 degrees: A, 14 hours. B, 22 hours. C, 19 hours. D, 20 hours. Arrangement B won on both measures, and the margin over A was 8 hours, which on a real lane is the difference between a compliant delivery and a claim.

Why the Top Two Packs Earn Their Place

Heat enters a box through all six faces, but the top face sees the highest external temperature because warm air stratifies in a trailer and sunlight lands on the lid. Inside, the air layer under the lid is the warmest part of the box. A pack on top of the payload intercepts that layer before it reaches the product, and it also suppresses convection, because cold air sinking from a top pack is what drives the internal circulation that keeps the payload uniform.

All-bottom placement does the opposite: it cools the bottom, creates a stable cold layer that does not mix, and leaves a warm cap sitting directly on the product. The payload centre may look acceptable at hour 12 while the top 30 mm is already at 10 degrees.

Why Side Placement Underperformed

Four packs standing against the walls present a small contact area to the payload and a large area to the box wall, which is the hottest surface they could be next to. They also leave the vertical centre of the box without cold. Unless the box is narrow, side packs mostly cool the wall they touch.

If side placement is unavoidable, lay the packs flat against the wall with the broad face inward, and always keep one pack on top. That combination recovers most of the gap in our tests.

The Insulation Mistake, and a Diagram That Survives a Night Shift

Wrapping packs in bubble wrap is a common attempt to slow the melt, and it does slow it, but it also slows the transfer of cold into the payload. The pack stays cold and useful-looking for longer while the product warms faster. Unless the wrap is on the outside face of the pack only, in which case it works as intended, full wrapping is a net loss.

The correct version of the idea is a thin bubble or foam sheet between the pack and the box wall, which slows heat ingress from the wall without insulating the pack from the product.

Draw the box in section, label each layer with a number, and print it on the inside of the lid. Use three words per pack position, not a paragraph. Photograph a correctly packed box and pin it at the bench. Temporary labour follows pictures; they do not read SOPs at 2 a.m.

Then verify once: pack three boxes exactly per the diagram, add a lid logger, and run them through the worst lane you have. If the lid logger stays within 2 degrees of the payload, the diagram is right and can be audited.

Placement only works if the pack keeps its shape long enough to be where you put it. A gel that slumps under stack load migrates to the bottom corner regardless of the diagram, which is why high-AUL SAP for cold-chain packs is the other half of this fix.