What Is a Desiccant? Types, How It Works and How to Calculate the Dose

Reviewed and updated by ICD - Week 42/2026

A desiccant is a material that can adsorb and hold water vapor in an enclosed space, used to prevent mold, rust and damage to goods during storage and transport. It is the most common and cheapest moisture-protection solution in packaging, and is especially important for export goods shipped by sea, where humidity and temperature swings can ruin a whole shipment even when it was carefully packed.

ICD Vietnam supplies container desiccants and Absortech desiccant packets: wall-hanging bags, blankets laid on top of the cargo and packets placed inside cartons, with calcium chloride that forms a gel and absorbs over 300% of its weight.

What a desiccant is and how it absorbs moisture

A desiccant works by adsorption: the surface of the material has countless microscopic pores that “capture” water molecules from the air, pulling the relative humidity (RH) inside a sealed package down to a safe level (usually below 50 to 60% RH).

When humidity falls below the dew point, water vapor no longer condenses into droplets on the surface of the goods, and that is how a desiccant prevents mold and rust.

One gram of silica gel has an internal surface area of up to 500 to 800 m², so a small amount can absorb a lot of water. Absorption capacity is expressed as a percentage of weight: for example, silica gel absorbs 20 to 35% of its own weight under ordinary conditions.

Container rain: why goods still get damp when sealed

Many businesses load dry goods yet they arrive moldy or rusty. The cause is “container rain”: during sunny, hot days the moisture in wooden pallets, cartons and goods rises into the air; at night the container walls cool down and water vapor condenses into droplets that drip onto the goods like rain.

A long sea voyage can repeat this cycle dozens of times.

A desiccant absorbs this water vapor before it condenses, so it is the main line of defense against container rain, and works better when combined with floor lining and sealed wrapping of the cargo.

Two kinds of condensation, and why the route decides the risk

The marine insurer Steamship Mutual distinguishes two kinds of condensation in its guidance on sweat. The guidance is written for ship holds, but the dew point mechanism is the same for a steel container.

Ship’s sweat (condensation on the walls): humid air inside is cooled below its dew point by cold steel, and water forms on the walls and roof and drips onto the goods. It occurs when goods are loaded in a warm, humid place and travel to a colder region.

Cargo sweat (condensation on the goods): the reverse. Warm, humid air meets goods that are still cold and condenses straight onto their surface. It occurs when goods travel from a cold region to a warm, humid one.

A desiccant lowers the humidity of the air inside the container, which pulls the dew point down so condensation is harder to trigger. It does not replace dry goods and sealed packaging, because water already inside the wood, cartons or goods keeps evaporating.

Common types of desiccant

Each type has a different absorbing ability and cost; choose according to the environment and transport time:

Note: silica gel and clay absorb a smaller percentage but hold moisture well at high RH; CaCl2 absorbs many times its own weight through a gel-forming reaction, and is the number one choice for containers.

A detailed analysis of each type and how to choose by goods is in types of desiccant: silica gel, clay, calcium chloride. Representative ICD products: the AbsorGel Hanging-C container desiccant bag for containers, the Pouch-S calcium chloride desiccant packet for cartons and the AbsorTerra clay desiccant packet in plastic-free kraft paper.

How to calculate how much desiccant to use

The amount of desiccant depends on the volume of the space, how airtight the packaging is, the starting humidity and the transport time. Two reference ways to calculate:

By the volume of sealed packaging: about 1 to 2 desiccant units for every 0.03 m³ (1 standard unit is about 33 g of silica gel absorbing about 6 g of water at 40% RH). For example a 0.3 m³ carton needs about 10 to 20 units.

By sea container: Absortech, the maker of the AbsorGel bags, publishes reference levels by voyage time and by how damp the goods are. The humid-cargo row is for timber, agricultural products and textiles that are not fully dry.

In terms of calcium chloride weight, a 20ft container needs about 5 to 11 kg and a 40ft container about 9 to 14 kg. These are the manufacturer’s reference levels; the actual amount also depends on climate along the route, the volume of air in the container, the packaging and the type of goods, and ICD advises the quantity for each shipment.

The more airtight the packaging, the less is needed. The precise dose by voyage and the formulas are in how to use desiccant packets correctly.

Choosing an Absortech product by where it goes

The active material is the same gel-forming calcium chloride, but each form suits a different position:

The calcium chloride bags are single use, one bag for one voyage. An unopened box keeps for up to 3 years in a warehouse below 60% humidity at 5 to 25 °C. Open the wrap only just before hanging, because the bag starts absorbing as soon as it leaves the wrap.

How to place desiccant correctly

Place desiccant as soon as the package is sealed, and do not leave it open for long because the beads absorb ambient moisture before they go to work. Distribute it evenly around the goods and place it near the most moisture-sensitive surface. In a container, hang the desiccant bags along both walls and behind the door, where condensation is strongest.

Combine it with a moisture-proof floor lining and sealed wrapping (PE film, aluminum bags) so the desiccant works at its best. For metal goods, use it together with VCI anti-corrosion.

Six checks before closing the container door

• Check the moisture of the materials: pallets, cartons and goods should be dry; let them dry or dry them before loading if needed.
• Wrap moisture-sensitive goods tightly: PE film or aluminum bags; add VCI for metal goods.
• Identify the route and number of days: warm and humid to cold or the reverse, about 30 or 60 days, to pick the right row of the dose table.
• Open the wrap of the desiccant bags only when you are about to hang them.
• Hang along both walls and behind the door: leave a gap of about 25 mm between the goods and the bag so the goods do not press and tear it.
• Photograph the hanging positions and record the number of bags: to compare with the result after the voyage.

Common mistakes when using desiccant

First, using too little for the volume and transport time: a desiccant that saturates mid-voyage stops working. Second, loading goods and wooden pallets that are still damp: the moisture inside is what causes container rain, and desiccant cannot carry that load.

Third, placing desiccant in packaging that is open, so the beads absorb outside air instead of protecting the goods. Fourth, using ordinary silica gel for a long sea voyage when a strongly absorbing CaCl2 type should be used.

Other names for desiccant (Vietnamese, English, Chinese, Japanese)

The same material has many names depending on where it is used. In Vietnam, “hat hut am” usually means loose beads (silica gel, clay), while “goi hut am” or “tui hut am” means packaged beads. When searching documents or working with overseas suppliers, the equivalent terms are:

Reading the result after the voyage

When the container is opened at the destination, the calcium chloride bags show whether the dose was enough. This is a practical reading, not a standard:

• Bags have swollen into nearly full gel and the goods are dry: the dose was right, keep it for the next shipment.
• Bags are full but the goods are still damp: the dose was too low or the goods carried too much moisture; raise the dose or use the humid-cargo row of the table.
• Bags are almost untouched but the goods are still damp: the cause is in the goods and packaging (water already in the wood or cartons, open wrapping), not a shortage of desiccant.

What the different desiccants are made of

Choose an article by need

• Using correctly, dose, reuse, safety: how to use desiccant packets correctly, how to reuse desiccant beads, what happens if you swallow a desiccant packet
• Importing and buying: HS code for desiccant, where to buy desiccant packets and what they cost
• Color-changing beads (blue, pink, orange): color-changing desiccant beads
• Food and mooncakes: food-grade desiccant packets, oxygen absorber vs desiccant packet
• Wardrobes, shoes, damp homes: desiccant packets for wardrobes, shoes and handbags, prevent mold and damp in home and wardrobe, ideal indoor humidity level
• Cameras, electronics, safes, guitars, cigars: desiccant packets for cameras, electronics and instruments
• Cars and foggy headlights: car dehumidifier bags for headlights and windshield
• Transformers, air compressors, insulated glass: industrial desiccants
• Drying flowers: drying flowers with silica gel
• Measuring and judging humidity: how to measure humidity, safe humidity for storing goods
• Hot, wet or leaking packets: desiccant packet hot, wet or leaking

Also see what is VCI anti-corrosion for metal goods and air dunnage bags for securing cargo inside containers.

Frequently asked questions

Which desiccant is best?

It depends on the purpose: silica gel is the cleanest and most common for components, electronics and pharmaceuticals; for long sea containers use CaCl2 container desiccant because it absorbs many times its own weight.

How much desiccant does a 20ft container need?

By the Absortech manufacturer table, a 20ft container needs from 5 bags of 1.0 kg (about 30 days, dry cargo) to 11 bags (about 60 days, humid cargo), which is roughly 5 to 11 kg of calcium chloride. A 40ft container needs 9 to 14 bags of 1.0 kg. Timber, agricultural products and damp textiles use the humid-cargo row of the table.

Why do sealed goods still get moldy when exported?

Because of container rain: day and night temperature differences make moisture condense and drip onto the goods. You need enough desiccant plus sealed wrapping and a floor lining.

Why do goods imported from Europe to Vietnam show water when the container is opened?

It is usually cargo sweat: when the door opens, warm and humid air meets goods that are still cold and condenses on their surface. To reduce it, keep the goods in sealed wrapping until they warm up and do not unwrap at the port straight away.

How long can a calcium chloride desiccant bag be used?

Each bag is single use, for one voyage. An unopened box keeps for up to 3 years in a warehouse below 60% humidity at 5 to 25 °C; open the wrap only just before hanging.

Can silica gel be reused?

Yes. Drying silica gel at about 120 °C for a few hours allows many reuses; clay and CaCl2 are usually single use. Temperature, time and which types not to dry are in the article on how to reuse desiccant beads.

Is desiccant poisonous?

Silica gel is inert and not poisonous, but it is not for eating; packets usually carry a warning. CaCl2 produces a solution after absorbing and must be kept off the goods. For accidental swallowing or eye contact the handling depends on the type; see what happens if you swallow a desiccant packet.

Should desiccant be used together with VCI?

For metal goods use both: VCI forms a rust-preventing vapor layer on the surface and the desiccant pulls general humidity down, two mechanisms that complement each other.

Contact ICD Vietnam for desiccant advice

ICD Vietnam Industrial Production Co., Ltd.

North: 3rd Floor, Thang Long A1 Building, Bau Hamlet, Thien Loc Commune, Hanoi, 0983 797 186 / 090 345 9186 / 090 5859 186
South: 551/212 Le Van Khuong, Tan Thoi Hiep, District 12, Ho Chi Minh City, 098 6784 186

Email: sales@icdvietnam.com.vn
Zalo: Chat on Zalo

References

• Institute of Packaging Professionals (IoPP)
• Steamship Mutual, The problem of sweat (ship’s sweat and cargo sweat)
• Absortech, AbsorGel and AbsorTerra Technical Product Sheets

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