The Complete Guide to Dunnage: What It Is, Why It Matters and How It Protects Your Supply Chain

Dunnage is the material that fills the empty space in your shipment so cargo stays put. It is not the box around your product. It is the system that keeps that box from moving inside the trailer, container or railcar. Wood, air bags, foam, corrugated panels and void fill are the most common types, and each one solves a different protection problem. This guide covers all of them, how they fit into the complete unit load and how to choose the right one for your operation.

If you have ever opened a damage claim and wondered what went wrong between the loading dock and the destination, the answer often comes down to one thing: the empty space inside the trailer that nobody filled. That is the problem dunnage solves.

Dunnage could mean the wood blocks bracing a steel coil to the trailer floor. It could show up as the air bag wedged between two pallet rows inside a container. Even the foam insert cradling a medical device so precisely that it cannot shift even a millimeter counts. Walk through any well-run warehouse and you will see these materials everywhere, doing their job quietly. Most people never think about them until they are missing and the fallout is devastating. 

What Is Dunnage?

Dunnage: Definition

Dunnage is any material placed around, under or between goods to secure, support and protect them during storage and transportation. Its primary purpose is to fill void space so cargo cannot shift, absorb impact so products are not damaged by vibration or collision and separate items so they do not contact surfaces or other goods that could cause harm.

The distinction between dunnage and packaging is important. Packaging encloses and protects an individual product: the box, the shrink wrap, the inner cushioning, whereas dunnage protects the load as a whole. It operates at the level of the pallet, the trailer and the container, filling the space between packages so nothing moves once the doors close.

This means a carton can be perfectly packaged and still arrive damaged if the open space beside it inside the trailer was not filled properly. The carton was never the problem. The empty space was.

Products used to manage that empty space take many forms. Dunnage packaging is versatile, from dunnage wood and dunnage air bags to dunnage void fill panels, foam inserts and corrugated separators. The right choice depends on the cargo, the transit mode and the specific risks the shipment will face. So, let’s take a closer look at the types of dunnage and the logic behind them. 

Types of Dunnage in Modern Shipping and Logistics

Clearly, not all dunnage does the same thing. The material that braces a steel coil to a flatbed is solving a completely different problem than the foam cradling a medical device inside a carton. Understanding what each type does best helps you match the right protection to the cargo you are actually shipping.

Wood Dunnage

If you ship heavy or industrial freight, wood is likely already part of your operation. Dunnage wood includes lumber, blocks and bracing pieces placed under, beside or between cargo to stabilize loads and prevent movement during transit.

Wooden dunnage is the standard for flatbed shipments, machinery, steel coils and construction materials. If you have worked with open-deck carriers, you know that most drivers carry six to ten pieces of wood dunnage at all times, cut in standard sizes from two-by-fours to four-by-sixes. 

Dunnage boards, the flat pieces of lumber placed under cargo to create clearance, elevate your goods two to six inches off the deck. That clearance gives forklifts access and keeps products away from moisture on the floor.

If your freight crosses international borders, this is where compliance becomes critical. All wood dunnage must meet ISPM 15 standards, meaning it must be heat-treated, debarked and stamped with the IPPC mark. Non-compliant shipping dunnage is one of the most common reasons shipments are held or rejected at destination ports, and that is a costly surprise you do not want on a tight delivery timeline. 

Millwood supplies ISPM 15 certified dunnage wood and packaging lumber, so compliance is built into the material before it reaches your facility.

ISPM 15 Definition

ISPM 15 is the international phytosanitary standard governing wood packaging material in global trade. It requires all raw wood dunnage to be heat-treated to a core temperature of 56°C for a minimum of 30 minutes, debarked and marked with a standardized IPPC stamp identifying the country, treatment facility and treatment type. Every piece of wood dunnage must carry this mark individually.

Dunnage Air Bags

If you have ever loaded a trailer and noticed gaps between pallet rows that nothing solid can fill, dunnage air bags are designed for exactly that problem. These inflatable cushions are placed between loads, against container walls or in the voids between cargo blocks to prevent lateral movement.

The process is simple: position the bag in the void and inflate it with a dunnage air bag inflator tool. The bag expands to fill the gap and locks adjacent loads in place. Because they are lightweight and compact before inflation, dunnage air bags add minimal tare weight to your shipment and take up little storage space when not in use.

Dunnage air bags come in several material types, each suited to different environments:

• Paper dunnage bags: Best for single-use dry freight where moisture is not a concern
• Polywoven dunnage bags: More durable, moisture-resistant and suited for ocean containers or humid conditions
• Vinyl dunnage bags: Built for reuse in repeating lanes where the same bags cycle back through the supply chain

One thing to keep in mind: sizing the bag to the void matters. An undersized bag leaves room for movement. An oversized bag cannot inflate fully enough to create reliable pressure. Getting the fit right is the difference between a bag that holds and one that does not.

Void Fill and Corrugated Dunnage

When your cargo needs separation and stability but not the weight of wood, dunnage void fill is often the answer. The category covers several distinct products, each designed for a specific job:

• A-frame void fill panels: Foldable corrugated panels that stand upright between pallet rows to block lateral movement in trailers
• Bulkhead panels: Large, rigid panels placed at the back of a partial load to prevent cargo from shifting forward during braking
• Cargo separators: Lightweight dividers placed between different product groups or pallet blocks to keep them from making contact
• Corrugated inserts and layer pads: Flat sheets placed between stacked layers or between individual products inside a carton to prevent item-to-item contact

All of these options are lightweight and recyclable, which makes them a practical choice when you need structure without adding significant weight to the shipment. If your loads are experiencing strap cut-in or corner crushing, edge protection solutions like corner protectors work alongside void fill to distribute pressure across the load rather than concentrating it at contact points.

If you ship bottled products, glassware or consumer goods where even minor surface damage affects saleability, corrugated inserts and layer pads are standard for a reason: they are one of the simplest and most cost-effective ways to eliminate contact damage inside a carton.

Foam Dunnage

If your products are fragile, high-value or irregularly shaped, foam dunnage gives you the most precise protection available. It can be cut or molded to match the exact contours of your product, cradling it in place and absorbing shock and vibration throughout transit.

Electronics, medical devices, precision instruments and aerospace components are common applications. The close fit between foam and product means virtually no movement is possible, which is why foam is the preferred choice when even a minor shift could result in damage worth thousands of dollars.

Foam costs more per unit than most dunnage types, but consider the math: the cost of one custom foam insert is a fraction of the cost of a single damage claim on a high-value product. And if you run a closed-loop supply chain where the same products travel between the same facilities, custom dunnage inserts made from foam can be reused across hundreds of shipments, driving per-trip cost down significantly.

Custom and Reusable Dunnage

Not every shipment fits a standard solution. If your operation has unique products, specific routes or handling requirements that off-the-shelf dunnage cannot address, custom dunnage designed from plastic, metal, fabric or composite materials may be worth exploring.

Reusable plastic trays are common in automotive manufacturing, where the same parts move between plants on predictable schedules. Fabric dunnage offers flexibility for irregular shapes and collapses flat for return shipping. Steel and engineered supports handle the heaviest loads where wood alone is not sufficient.

The question to ask yourself is about volume and repetition. For a one-time shipment, standard materials work well. But if you are shipping the same product hundreds or thousands of times per year on the same route, a custom reusable system often pays for itself through reduced material waste, lower damage rates and faster packing times.

Dunnage Types at a Glance

How to Choose the Right Dunnage for Your Operation: Consider the Complete Unit Load

Knowing the types of dunnage is only one half of the picture. The other half is understanding how dunnage connects to everything else that protects your cargo. No single material, no matter how well chosen, does the job alone.

Dunnage is one component of a larger system that includes the pallet, the strapping, the stretch wrap, the edge protection and the container itself. This is the full unit load: the complete assembly of product, packaging and securement that moves as a single entity through the supply chain.

Understanding how dunnage fits into this system is what separates entry-level freight protection from optimized freight protection. Dunnage fills the space cargo could move into. Strapping removes the cargo’s ability to move by anchoring it to the pallet. Wrapping contains the entire assembly. Together, they eliminate movement from every direction to protect your products. 

A load with dunnage but no strapping is filled but not anchored. A load with strapping but no dunnage can still rock within unfilled gaps. The combination of both, designed and tested for the specific cargo and transit conditions, creates a load that travels as one immovable unit.

This systems-oriented thinking is central to how Millwood operates as a strategic provider across industries. Because dunnage spans across lumber and panels and load securement and protection, it touches nearly every product category in the unit load. That breadth matters. When your transit partner understands how every component interacts, the result is a system that works together rather than a collection of parts that happen to share a trailer.

Industry standards reinforce this approach. The International Safe Transit Association (ISTA) and the National Wooden Pallet and Container Association (NWPCA) have both established guidelines for how unit load systems should be designed and tested. Millwood’s ISTA-certified Packaging Science Lab validates solutions under simulated shipping conditions that match your operation and align with these guidelines, so you can see how dunnage, strapping and wrapping perform together for your products under real-world stress rather than guessing in the field.

When you work with Millwood to find the right dunnage solution as part of the whole unit load, these are some of the common factors that are considered: 

• Cargo weight and fragility determine whether the priority is structural support or shock absorption
• Transit mode and distance influence how much vibration, shifting and environmental exposure the cargo will face
• Compliance requirements dictate whether wood dunnage needs ISPM 15 certification for international shipment
• Volume and frequency determine whether reusable or disposable dunnage is the better economic choice
• Sustainability goals influence material selection and end-of-life recovery options. Recyclable corrugated panels, reusable custom systems and material reduction strategies all connect dunnage decisions to broader sustainable supply chain management goals

There are a lot of factors that go into finding the right solution to keep your product safe, but the core principle is simple: dunnage is one piece of a larger protection strategy, and the pieces in that strategy work best when they are designed together.

The Invisible System That Keeps Your Supply Chain Moving

Dunnage is easy to overlook. It is not the product your customers see. It is not the truck, the warehouse or the pallet. But it is the reason those things work together without failure or waste. Every shipment that arrives intact, every damage claim that you don’t have to file, every product that reaches a customer in the condition it left the facility: dunnage played a role.

The details of getting it right, from choosing the correct material to sizing it for the void to ensuring ISPM 15 compliance, can feel like a lot to manage on your own. That is exactly where a partner who understands the full unit load system makes the difference. Millwood combines 40 years of packaging expertise with ISTA-certified testing to help you find the right dunnage fit for your operation, so you can focus on moving product and let the protection take care of itself.

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