Humidity is the tax nobody on the Gulf Coast budgets for
Corrugated loses roughly half its compression strength at 90% relative humidity. In Houston that is most of the year. What that means for wall count, stacking and how long a box survives.
By Marisol Vega, Grading line lead · 6 min read · Published October 24, 2023
Have a question this raised?
Tell us what you have or what you need. A real person in Houston reads every one of these — usually within a couple of hours during dock time.
- The relationship, in round numbers
- Recoverable versus permanent
- Where the water actually comes from
- What to do about it
- The seasonal pattern we see on the yard
Corrugated retains about 50% of its dry compression strength at 90% relative humidity and about 70% at 70% RH. Houston sits above 70% RH for most of the year, so any published load rating needs a 0.7 multiplier for non-climate-controlled storage — and moisture damage, once it happens, is permanent even after the box dries.
- Category
- Houston & Texas
- Published
- October 24, 2023
- Written by
- Marisol Vega, grading line lead
Every packaging engineer knows corrugated is hygroscopic. Very few purchase orders reflect it. On the Gulf Coast that gap is expensive, because our climate does not give the board a break between May and October.
The relationship, in round numbers
| Relative humidity | Approx. strength retained |
|---|---|
| 50% (climate controlled) | 100% — the reference condition |
| 70% | ~ 70% |
| 80% | ~ 60% |
| 90% | ~ 50% |
Houston's average relative humidity runs in the 70s year-round and spends a lot of summer mornings in the 90s. A box rated 2,000 lb in a laboratory is realistically a 1,400 lb box in an open dock here, and closer to 1,000 on a bad August morning.
Recoverable versus permanent
Here is the distinction that matters operationally. Strength lost to ambient humidity comes back when the air dries out — the fibers swell and shrink, and the board largely recovers. Strength lost to liquid water does not.
A box that stood in a puddle carries a tide line: a wavy discoloured band a few inches up the wall. Even bone dry, that box has lost strength permanently, because the fibers swelled, the glue lines stressed and the flute geometry never returned to where it started. On our grading line a tide line is an automatic downgrade regardless of how the box feels in the hand.
Where the water actually comes from
- The slab. Concrete wicks ground moisture upward, day and night. Always on a pallet.
- Condensation. A cold load brought into a warm humid dock sweats. So does a box moved from a cooler.
- Dock doors. Every open door in July is a humidity exchange with the outside.
- Roof leaks and drips nobody has walked the racks to find.
- Wash-down. Floor cleaning that runs under a pallet is invisible until you lift it.
What to do about it
Two categories: cheap habits, and spec changes. Do the habits first, because they are free and they work.
- Everything on pallets. No exceptions, not even for an hour.
- Knock down empties within 48 hours. A standing empty in a humid dock is a sponge with a shape.
- Rotate stock. Boxes should not sit for a season waiting to be used.
- Lids on anything stored near a dock door, even indoors.
- For long storage, wrap the pallet — not to hide the boxes, but to slow the moisture exchange.
If the habits are already right and boxes are still failing, then the spec is genuinely under-built. Apply the 0.7 multiplier to your calculated load and see whether it pushes you up a wall count. Often it does, and often that is cheaper than the failures.
The seasonal pattern we see on the yard
Grade A intake drops noticeably in late summer. The same customers sending the same boxes get more Grade B calls in September than in February, and it is not because their people got careless — it is because the air changed. Buyers who plan around that pattern place their large orders in winter and store dry, which is one of the few genuinely free savings available in this business.
- ECT
- Edge Crush Test. Pounds per linear inch a board edge withstands before buckling. The best single predictor of stacking performance.
- Flute
- The wavy medium between liners. Its arch geometry is what gives corrugated its strength-to-weight ratio.
- Grade (box)
- A condition classification for used boxes. 1A new, A one clean use, B up to two repairs, C multiple repairs, then scrap.
- Tide line
- A wavy discoloured band showing a box stood in water. Permanent strength loss even after drying.
- Wall count
- The number of fluted layers. 2-wall through 5-wall in common use.
Marisol Vega — Grading line lead
Written on the yard at 1050 E Richey Rd, Houston. Everyone who writes here works the grading line, the baler, the routes or the quote desk — there are no guest posts and nothing on this blog is syndicated.