Coated vs Uncoated FIBC: Do You Need a PE Liner for Sea Freight?
Published 5 November 2026 · BULKFORTE
A coated FIBC has a thin PP film on the fabric that reduces sifting and light moisture, but it is not waterproof. A PE liner is a separate film inside the bag and is the main moisture barrier. For hygroscopic products shipped by sea to humid markets, use a sealed PE liner, a dry container and minimal headspace, because container condensation can wet cargo whatever the outer fabric.
On this page
The short answer
Coating and liners are two different tools. Start from your product, then adjust for the route:
- Product not sensitive to moisture: uncoated fabric is usually enough.
- Fine or sifting product: coated fabric, because it closes the weave.
- Hygroscopic product or a long humid route: a PE liner, with a coated outer as an option.
Neither stops container rain on its own. The real fix is a bag spec, a filling method and a loading plan that fit your route.
Coated vs uncoated FIBC: what the coating actually does
Uncoated woven polypropylene has tiny gaps in the weave, so air and moisture pass through it. That suits products that need to breathe. Coating bonds a thin film to the fabric and seals those gaps.
The side matters, so name it on your order:
- Inside coating is the most common. It also stops powders sticking in the weave during discharge.
- Outside coating faces rain and outside humidity.
- Two-side coating does both and costs more.
What coating does not do is seal the bag. It slows moisture movement rather than stopping it, and it will not keep liquid out. Treat a coated bag as moisture-resistant, never as a sealed barrier.
Telling them apart by hand: press the weave together and try to spread it, on both the outside and the inside of the bag. If the weave will not spread apart, the fabric is probably coated.
Source: National Bulk Bag, "How to choose between coated and uncoated FIBC" (blog.nationalbulkbag.com); PackTonne, "Coated vs uncoated FIBC fabric" (packtonne.com); Codefine, "Why moisture damage happens" (codefine.com).
What a PE liner does (and why it is not the same as coating)
A liner is a separate polyethylene film inside the bag. Its protection depends on its fit and its closure.
- Tubular: a simple tube. It works well in controlled conditions but is more prone to shifting during filling.
- Form-fit: shaped to the bag, so it keeps closer contact with the walls and corners.
- Closure: a heat-sealed closure gives near-airtight containment. A tied closure allows vapour exchange over long storage.
Thickness should be agreed for your product. As an example of published figures, one Vietnamese supplier quotes coating at 25–30 microns and liners at 50–150 microns. Those are its numbers, not a rule, and not a promise of what any other supplier offers.
Source: Codefine (codefine.com); EPP Vietnam, "When should we use PE liners" (epp.vn).
Searching for a "moisture proof jumbo bag"?
That is what many buyers type, but no fabric makes that promise true by itself. What you can buy is a moisture-resistant construction: the right fabric, a liner with a good closure, and careful loading. Ask suppliers which of those they are actually quoting.
Container rain: why sea freight changes the decision
How condensation forms in a container
Warm air inside a closed container holds moisture. When it cools, often at night or as the ship crosses into a cooler climate, that moisture condenses on the steel and can drip onto the bags below. Shippers call this container rain. Moisture can also rise from below: it can wick up through pallet boards from a damp floor.
The same effect can happen inside the bag. If humid air is trapped above the product, a temperature change can turn it into liquid water there too, which is why headspace matters.
Source: EPP Vietnam (epp.vn); Codefine (codefine.com).
Routes where it matters most
Risk rises with temperature swings, humidity and time in the box. Be careful with cargo from China to West and East Africa, Southeast Asia, the Gulf and Australia, especially in a rainy season or with transhipment, when containers may also wait in open yards.
Decision table: product and route to construction
Use this as a starting point, not a standard. Confirm it with your own product, your filling line and any past claims.
| Product | Short, dry route | Long, humid sea route |
|---|---|---|
| Aggregates, sand | Uncoated | Uncoated; coated if the product has fines |
| Granular fertilizer | Coated | Coated + liner |
| Urea, other hygroscopic fertilizer | Uncoated + liner | Coated + sealed liner |
| Sugar, salt | Uncoated + liner | Coated + sealed liner |
| Cement, fine powder | Coated | Coated + liner |
| Resin pellets | Uncoated | Uncoated + liner if the grade is moisture-sensitive |
| Chemicals | Coated; liner per the product's data sheet | Coated + sealed liner, per the data sheet |
If your product is dangerous goods, the required UN code decides the construction, not this table. A UN test report covers only the bag specification that was tested.
What to write on your PO
"Moisture-resistant bag" is not a spec. Write down each of these:
- Fabric: coated or uncoated and, if coated, which side.
- Liner type: none, tubular or form-fit.
- Liner thickness: the figure agreed for your product.
- Attachment: glued, sewn, tabbed or loose.
- Closure: heat-sealed or tied, and who seals it after filling.
- Spouts: fill and discharge, and how the liner fits them.
- UV stabilisation if bags will be stored outdoors.
- Safety factor and safe working load.
- Test report: if you rely on one, check the test report covers your liner spec.
Our bag specification section shows where the fabric and liner sit in the construction.
It's not only the bag: loading checklist for humid routes
- Use a dry, clean, undamaged container. Check for light through the roof and walls, and for a damp floor.
- Do not fill warm product and seal it straight away. Let it cool first, so less moisture is trapped.
- Keep headspace in the liner to a minimum before sealing.
- Keep bags off a damp floor. A slip sheet between pallet and bag can help.
- Consider desiccants, a container liner or kraft paper as options, based on your cargo and route.
- Ask for photos and video of the container before, during and after loading.
Source: Headspace, desiccants, slip sheets and container liners: Codefine (codefine.com). Other points are common loading practice; follow your forwarder or insurer where their advice differs.
How to check you got what you ordered
- Do the hand test on sample bags, inside and outside.
- Ask for a sample photo that shows the liner in place and how it is closed.
- Check shipment photos and video against your PO, line by line.
- On arrival, look at the top layer of bags and the container ceiling for water marks, and photograph them before unloading further.
For the full list, see our pre-shipment inspection guide.
How BULKFORTE specs bags for humid routes
We go through fabric, coating, liner, spouts and safety factor with you before production, and PE liners can go in the same order as your bags.
We supply FIBCs through contracted manufacturing partners in China. Production, inspection and loading take place at their facilities, and buyers are welcome to audit any factory on site.
Our team brings 20+ years making FIBCs. Shipment photos and video are available on request, and we reply within 12 hours (GMT+8).
FAQ
Is a coated FIBC waterproof?
No. Coating closes the gaps in the weave and slows moisture, so a coated bag is moisture-resistant at best. It will not keep liquid out, and water can still enter at seams, spouts and the top closure.
What is the difference between coated and laminated bulk bags?
On most quotes they mean the same thing: a thin plastic film bonded to the woven fabric. Ask the supplier which side is coated and whether the whole bag or only some panels are coated, then write the answer on your PO.
When do I need a PE liner in a jumbo bag?
Use one when the product absorbs moisture, such as urea, sugar or salt, when a fine powder must not escape or pick up contamination, or when the bags will spend weeks at sea on a humid route. Sand and most aggregates usually do not need one.
Can a coated bag replace a liner?
Not for moisture-sensitive cargo. Coating slows moisture movement but does not stop it. A liner is a separate film that can be sealed, so it is the main moisture barrier. Coating and a liner do different jobs and are often used together.
What is container rain?
It is condensation inside a shipping container. Warm, moist air in the container cools, often at night or when the ship enters a cooler zone, and water forms on the steel roof and walls, then drips onto the cargo.
Should the liner be glued, sewn or loose?
It depends on how you fill and empty the bag. A form-fit or attached liner stays in place better during filling and discharge. A loose liner can shift during filling or pull out during discharge. Agree the attachment with your filling line in mind.
Do liners affect discharge or the safety factor?
They can affect both filling and discharge: the wrong liner can trap air, slow discharge or pull out with the product. The safety factor is proven by testing a specific bag construction, so tell the supplier about the liner before a test or drawing is agreed.
Does a UN test report cover a bag with a different liner?
No. A UN test report covers only the bag specification that was tested. A different liner can mean a different construction, and UN codes separate lined from unlined bags, so check the liner in the report against your order. How to verify a test report.
Do desiccants replace a liner?
No. Desiccants take moisture out of the air around the cargo and only until they are saturated. A sealed liner protects the product itself. On humid routes they work together, along with a dry container and minimal headspace.
Tell us your product, port and transit
Send your product, destination port and expected transit, and we will suggest a construction to check against your own experience. Get a quote.