Moisture Absorber Containers FAQS

Condensation and moisture accumulation inside shipping containers is an extremely common problem, often referred to as ‘container rain’ or ‘container sweat’. It occurs because containers experience significant temperature fluctuations — the metal structure heats rapidly in sun and cools quickly at night — causing the air inside, which always contains water vapour, to reach its dew point and deposit moisture on the container walls, ceiling, and cargo.
This is exacerbated when goods stored inside release moisture (wood, textiles, paper, and food products all do this), when the container is opened and humid air enters, or when the container sits in a particularly exposed, wet location. Moisture absorbers are an effective, cost-efficient tool for reducing the relative humidity inside a container and protecting whatever is stored within.

The amount of desiccant required depends on several factors: the size of the container (a 20ft container has roughly 33 cubic metres of air space; a 40ft has approximately 67 cubic metres); the duration of storage or transit; the ambient humidity at the storage location; and what is stored inside (organic materials release more moisture). As a general guide for a 20ft container in standard UK conditions, two to four large hanging desiccant bags (each with a capacity of around 1kg or more) typically provide adequate protection for one to three months.
For longer storage, more desiccant or a higher-capacity product is needed. For containers holding moisture-sensitive cargo, it is worth consulting a specialist — over-protecting with more desiccant than needed costs little, while under-protecting can result in expensive damage.

When properly sized and applied, moisture absorbers can significantly reduce the risk of moisture-related cargo damage. Common damage caused by container condensation includes rust and corrosion on metal components; mould and mildew on textiles, paper, and wood; blooming and contamination of food products; degradation of packaging; and damage to electronics, machinery, and other sensitive goods.
Desiccants work by reducing the relative humidity inside the container below the threshold at which condensation forms, typically targeting below 60% RH. However, desiccants alone may not be sufficient in extreme conditions — for highly sensitive cargo or very long transits, combining desiccants with barrier packaging (such as desiccant-lined container liners) provides the most comprehensive protection. Regular inspection and replacement of desiccants is also important.

Standard desiccant moisture absorbers are safe for use with the vast majority of goods. However, a few considerations are worth noting. Calcium chloride desiccants produce a brine solution as they absorb moisture; if a bag leaks or tips over, this liquid can cause corrosion on metal surfaces and damage to some materials, so proper hanging or contained units are important.
For food-grade storage, ensure you are using a food-safe desiccant product rather than an industrial grade. Some specialist cargo may have very specific humidity requirements — pharmaceutical products, for example, may need monitored, controlled conditions rather than simple desiccants. If you are unsure whether a particular desiccant product is appropriate for your cargo, consult the supplier before use.

Good ventilation and moisture absorbers are complementary rather than interchangeable strategies. Ventilation helps by exchanging humid air inside the container with outside air, which can reduce moisture build-up — most shipping containers have built-in vents for this reason. However, in the UK climate, outside air is frequently more humid than the air inside the container, meaning that simply ventilating can introduce rather than remove moisture.
Desiccants address moisture regardless of external conditions by actively absorbing water vapour from the air inside the container. For best results, ensure container vents are unobstructed (but consider the external humidity levels), place desiccants appropriately, and ensure goods are properly dried and packaged before storage. Combined strategies are usually most effective for long-term storage.

Beyond desiccants, there are several measures that significantly reduce moisture problems in shipping containers. Elevate stored goods off the floor using pallets or dunnage to allow air circulation and reduce contact with any moisture that collects on the container floor. Ensure all goods are completely dry before placing them in the container — even slight residual moisture in wood, cardboard, or fabrics will contribute to the internal humidity.
Check the container itself for any rust holes, damaged seals around the doors, or degraded floor boards that may allow water ingress directly. Consider lining the container with an insulating material, which reduces temperature fluctuations and therefore condensation formation. And if the container is in a particularly exposed location, ensuring good drainage and shade can also make a meaningful difference.

Yes — at Jennings Containers, we understand that protecting the contents of your container is just as important as the container itself. We can advise on appropriate moisture protection solutions for your specific storage requirements and point you towards suitable desiccant products.
Our team has extensive experience with containers used for a wide range of storage applications and can help you assess the right level of moisture protection for your situation — whether you are storing personal belongings, business stock, machinery, or building materials. If you are purchasing or hiring a container from us and have questions about moisture management, please ask when you get in touch and we will include practical recommendations.

 Some categories of goods are far more vulnerable to moisture damage than others and require particularly careful protection. Metal components, tools, and machinery are highly susceptible to rust and corrosion. Electronics can suffer permanent damage from even moderate condensation. Documents, books, and artwork are easily damaged by mould and dampness.
Textiles — including upholstered furniture, clothing, and bedding — readily develop mildew in humid conditions. Food products and organic materials are at risk of spoilage. Woodwork, including furniture and timber, can warp and crack with moisture cycling. If you are storing any of these categories in a shipping container, investing in adequate moisture protection is not optional — it is essential to ensure your goods arrive or emerge from storage in the same condition they went in.