Moisture-proof packaging is the most neglected — and most expensive — part of any sea-freight order. When a container is opened at destination to find mould spots, rusted metal trims and limp, curled hang tags, the fabric is rarely at fault: condensation inside the box did the damage. Such losses are usually not fully recoverable on insurance, so the cost lands on you.
This guide works through moisture protection layer by layer — garment moisture content, inner packaging materials, carton and full-container desiccant setup — compares the main desiccant types, and closes with a pre-packing checklist.
1. Why Garments Mould in Transit
Mould needs three conditions at once: moisture, a suitable temperature and a nutrient source (starch sizing and cotton fibre itself both qualify). Sea freight is very good at supplying all three.
- Container rain — the box heats by day and cools at night, so vapour condenses on the walls and the top of the load and drips onto cartons;
- Excess residual moisture — garments packed straight after pressing or drying without conditioning carry their own water into a sealed bag;
- Wet cartons and inserts — boxes that have already taken up humidity, or tissue and board that absorbed water in a damp warehouse;
- Long, multi-climate voyages — Asia-Europe and trans-Pacific runs take 25–40 days and cross several temperature and humidity zones.
2. Four Layers of Protection, From Garment to Container
| Layer | What to do | Best for |
|---|---|---|
| Garment | Confirm moisture equilibrium before packing; never pack hot garments | All categories, especially knits and heavy fabrics |
| Labels | Choose moisture-tolerant hang tag stock and keep tags inside the same bag | Sea freight, humid destinations |
| Inner packaging | PE bag plus desiccant, or a vented bag by category | Most exported garments |
| Carton | Control carton moisture content; line with barrier paper plus desiccant | |
| Container | Hang container desiccant bars; add barrier film if needed | FCL sea freight, rainy season, cross-climate routes |
3. Choosing a Desiccant: Four Options Compared
| Type | Capacity | Best for | Watch out for |
|---|---|---|---|
| Silica gel | Medium | Underwear, bags, shoeboxes and small items | Must be labelled do not eat; check sachet integrity |
| Clay (montmorillonite) | Medium to high | Standard carton configuration for garments | Cheap and light; stays solid when saturated |
| Calcium chloride | High | Long high-humidity voyages and high-risk categories | Turns to gel; must be leak-proof |
| Container desiccant bar | Very high | FCL sea freight, sized by route and season | Quantity depends on container type, route and packing humidity |
4. Barrier Bags vs Vented Bags: When Not to Seal
A standard PE bag stops liquid water but offers limited vapour barrier; genuine moisture blocking needs metallised or laminated barrier film. The opposite problem also exists: some categories must let internal vapour escape, and sealing them shut makes things worse.
- High-value garments, leather and cashmere — use foil or metallised laminate bags with desiccant, expel air and seal;
- Down jackets, fur and heavy wool — leave vents or use perforated bags so internal vapour can escape slowly;
- Styles with heavy metal trims — confirm rust protection on zips, eyelets and hardware and keep them off damp board;
- Timing the seal — let garments condition in the workshop before bagging; never seal straight from the press.
5. Seasonal and Destination Risk
- Southern China damp season (February to April): workshops and warehouses are humid, so inserts and cartons absorb water first — check before packing;
- Southeast Asia and tropical Americas routes: hot and humid throughout, so calcium chloride and container bars should be standard;
- Europe and North America in winter: large temperature swings drive condensation, so use barrier paper inside cartons together with container bars;
- Air freight is short and lower risk, but open-air loading in rain still needs rain covers and complete inner packaging.
6. Pre-Packing Checklist
- Garments have reached moisture equilibrium; nothing packed hot;
- Carton moisture content under control (commonly 12% or less) with no soft, damp boxes;
- Desiccant type, specification and quantity confirmed against container type and route;
- Whether container bars are needed, in what quantity and at which hanging points;
- Whether hang tags and care labels are moisture-tolerant and packed in the same bag as the garment;
- Ask suppliers for desiccant material data and safety sheets for destination customs and buyer records.
Related reading
- Garment export packaging requirements — how carton loading and moisture protection work together.
- How to specify vent holes in garment bags — which categories must breathe rather than be sealed.
- Eco garment bag options — the trade-offs and limits of recycled and biodegradable film.
- Back to the Trims FAQ for more articles on hang tags, woven labels, care labels and packaging.
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