Relative humidity is relative to temperature — which is why a container that was fine at noon produces condensation at three in the morning.
Relative humidity is the most used and least understood term in packaging. The word doing the work is relative.
What it measures
Air can hold a certain maximum amount of water vapour at a given temperature. Relative humidity is the actual moisture content expressed as a percentage of that maximum.
The critical point is that the maximum changes with temperature — sharply. Warm air holds far more water vapour than cold air.
This means the same absolute quantity of water can be 50% RH in the afternoon and 100% RH at dawn, with nothing having been added. Cool the air enough and it reaches saturation; cool it further and the excess condenses as liquid water.
That temperature is the dew point, and it is why a shipping container that seemed perfectly dry at loading produces condensation at three in the morning.
Why 50% is the working target
Most packaging specifications aim to keep relative humidity below 50%, and there is real reasoning behind the number rather than convention.
The damage mechanisms have thresholds:
- Steel corrosion effectively begins above roughly 60% RH at the metal surface
- Mould and fungal growth requires around 65% equilibrium relative humidity in the material
- Caking and physical change in powders and hygroscopic solids starts lower and varies by product
Holding below 50% keeps a margin under the two principal thresholds. It is not an arbitrary round number.
Why desiccant capacity depends on RH
This is where relative humidity stops being abstract and starts affecting your costs.
Silica gel adsorption is driven by the gradient between ambient humidity and the very low vapour pressure inside its pores. A steeper gradient means more uptake. Our published figures for non-indicating grade:
- 20% RH: ≥5% of its own weight
- 50% RH: ≥13%
- 90% RH: ≥70%
Fourteen times the capacity at the humid end. A desiccant is not a fixed-size container, and any dosing calculation that ignores the humidity it will actually work against will be wrong.
It also explains why silica gel is unsuitable below about 30% RH — there is barely any gradient left to drive adsorption. That is molecular sieve territory.
Equilibrium relative humidity
One further term worth knowing. Equilibrium relative humidity is the RH of the air immediately surrounding a material once the two have reached balance.
It matters because hygroscopic cargo does not merely sit in humid air — it creates it. Textiles, leather, timber, coir and agricultural commodities loaded with moisture will release it into the container until the air reaches equilibrium with them.
This is why a container of cotton goods generates its own condensation even in temperate conditions, and why textile shipments need container-level desiccant rather than sachets alone.
Measuring rather than assuming
Most warehouses we ask have never logged their humidity. It is worth doing: a cheap data logger left running for a week tells you whether your stock loss problem is a packing problem or a building problem.
The answer changes what you should spend money on. If the building runs at 75% RH through the monsoon, dehumidifying the space is often cheaper than over-dosing every carton for a season — and considerably more reliable.
