Mould and Mildew on Knitwear: Causes, Prevention and Damage Control

By YZR Sweaters · 2026-10

A musty smell on arrival is one of the few defects that can turn an entire order unsellable at once, and it is also one of the few that buyers and factories routinely blame each other for. In our experience the argument is usually settled by one question: was the garment dry when it went into the bag? Almost everything follows from the answer.

Why knitwear is more exposed than woven apparel

Mould needs four things: a food source, moisture, warmth and time. A knitted garment supplies the first one better than most:

• Cellulosic and protein fibres hold water. Cotton, viscose, linen, wool and cashmere are all hygroscopic. Wool can hold roughly a third of its weight in moisture without feeling wet to the touch, which is exactly the problem — a sweater can be measurably damp and still feel dry in the hand.
• Knitted structure holds air. Loops and ribs create thickness and trapped air, so a knit dries more slowly through the middle of a folded stack than a flat woven fabric does.
• Finishing adds water. Washing, softening, enzyme treatments and steam pressing all introduce moisture, and the garment goes from that process straight to folding. Our notes on garment dyeing, washing and finishing cover how much of the final hand feel depends on those wet processes.

Synthetics are not immune — a polybag with trapped humidity will spot an acrylic knit too — but natural fibres are where the damage concentrates.

The four ways it actually happens

1. Garments packed before they are dry

The most common cause, and entirely within the factory's control. A drying cycle gets shortened to hit a packing date, or a stack of steamed garments is folded while still warm. Warm plus damp inside a sealed polybag is close to ideal growing conditions, and the spots appear later — sometimes not until the carton is opened at the destination warehouse.

2. High humidity at the packing bench

In Dongguan the humid season runs roughly March to May, and again through the summer, with relative humidity regularly above 80% for weeks. A dry garment folded on a bench in that air picks up moisture from the atmosphere before it is even bagged. Factories that pack in a conditioned room during those months have far fewer problems than ones that pack in an open shed.

3. Container rain

Warm, humid air inside a steel container cools overnight, condenses on the roof and walls, and drips onto the cartons below. It is called container rain and it is a physics problem, not a cleanliness one. It is worst on routes with big day-night temperature swings and on containers that sit on a hot yard before loading. Our guide to ocean freight containers for sweaters covers stuffing and ventilation in more detail.

4. Storage at the destination end

Goods delivered in good condition can still go musty sitting in a non-climate-controlled warehouse through a damp winter. If your own storage runs above about 65% RH for extended periods, that is your risk to manage, not the factory's — which is worth knowing before you open a claim.

What prevention looks like on a real production line

None of this is expensive. It is mostly discipline:

• Dry to a standard, not to a feeling. Garments should be tumble-dried or passed through a drying tunnel and then rested until they reach ambient temperature before folding. Warm garments must never be bagged — condensation inside the bag is guaranteed.
• Check moisture before bagging. A pin-type moisture meter on a folded stack takes seconds. Experienced packers also just smell the stack; mustiness at the bench means the batch is not dry.
• Use vented polybags where the customer allows them. A few vent holes let trapped humidity escape. Sealed bags are sometimes required for other reasons — decide deliberately rather than by default.
• Dose desiccant by carton volume. A rough working rule is 20–40 grams of silica gel or clay desiccant per carton for knitwear, more for long transit or tropical routes. Clay desiccant performs better than silica at high humidity, which is why it is the usual choice for sea freight out of South China.
• Add container desiccant for long voyages. Container dry bags or desiccant strips hung along the walls, sized to the container, handle the condensation that carton-level desiccant cannot.
• Inspect the container before stuffing. Dry floor, no rust patches, no water staining, doors seal properly. A container with a leaking roof gasket will undo everything else.
• Do not load in the rain, and do not load a cold container into hot humid air. Both produce condensation.

Carton and packing specification is also part of this — board quality, liner and how tightly cartons are filled. See sweater packaging and shipping for the packing side.

How to check a shipment before you accept it

Mould is usually detectable at goods-in if someone is looking for it:

• Smell first. Open a carton from the middle of the container, not the one nearest the door. A musty, cellar-like odour is the earliest signal and it appears before spots do.
• Look at the carton. Water staining, soft or wavy board, rust marks from the container floor onto the carton.
• Check the desiccant. If it is saturated to a hard lump, the carton took on real moisture; colour-change indicator desiccant makes this obvious.
• Spot-check garments from the top and the bottom of several cartons, under the arms and inside folds, where moisture collects.

Build this into the inspection rather than doing it ad hoc. Our notes on pre-shipment inspection explain how to add a moisture and odour check to the AQL sample without slowing the inspection down.

If it arrives damaged: what is realistic

Be clear-eyed about outcomes:

• Light surface spotting and odour can often be recovered by airing thoroughly, then professional wet cleaning. On wool this usually means the garments are sellable, though not as first quality.
• Established mould inside the fibre — visible mycelium, persistent odour after cleaning, discolouration — is not reliably recoverable on knitwear. Trying to sell it usually produces returns and a worse problem than the write-off. See complaints and returns handling for how that conversation with your customer should go.
• Who pays depends on the Incoterm and on evidence. Under FOB the risk transfers at loading; under DDP or DAP the seller carries it to destination. Either way, photographs of the cartons at delivery, the moisture readings and a signed delivery note recording the exception are what make a claim go anywhere. Incoterms for knitwear shipping sets out where risk sits for each term.

One honest caveat: mould is the claim insurers most often dispute, because it can be argued as inherent vice or insufficient packing rather than a transit peril. That is another reason the prevention work matters more than the paperwork.

What we do about it

At YZR Sweaters we work with certified partner factories (BSCI, SEDEX, Oeko-Tex) across Dongguan, and during the humid months we treat drying and packing as a controlled step rather than the last thing that happens before the truck arrives: garments pass through a drying cycle and rest to ambient before folding, packing rooms are dehumidified, desiccant is dosed per carton and logged, containers are inspected and photographed before stuffing, and the pre-shipment inspection includes an odour and moisture check on cartons drawn from different parts of the load. If you are shipping into a humid market or your goods will sit in a warehouse before sale, tell us at order stage and we will increase the desiccant and consider a moisture-barrier liner for the cartons — it costs very little and it removes the most unpleasant surprise in the business.

Frequently asked questions