A warm deck structure places all insulation above the structural deck, creating a single continuous layer with no cold bridge between the insulation and the exterior. A cold deck structure separates the insulation from the waterproofing layer with a ventilated air gap. In sandwich panel terms, the core material, its density, and how the panel is sealed determine which thermal and structural behaviour you get. The sections below work through the practical differences across thermal performance, structural strength, moisture behaviour, and weight.
Note: This article covers the broader sandwich panel market. Our range at Compoform is limited to PP honeycomb and PET foam cores for commercial vehicles and scaffolding.
What materials make up a warm deck versus a cold deck structure?
A warm deck sandwich panel uses a continuous insulating core bonded directly between two structural face skins, with no ventilation gap. The core material, typically a closed-cell foam or honeycomb, carries both the thermal and structural load. A cold deck structure introduces a ventilated cavity between the insulation layer and the outer weathering surface, separating thermal and structural functions across distinct layers.
In a warm deck panel, the face skins and core work as a composite unit. Common core materials in the broader market include polypropylene foam, PET foam, and PP honeycomb. The skins are typically glass-fibre-reinforced thermoplastic, though steel and aluminium are also used in conventional construction. The bond between skin and core is what transfers shear loads and gives the panel its stiffness.
A cold deck assembly is more of a layered system than a single panel. The structural deck sits below, insulation boards are laid on top, and a ventilated air gap separates these from the outer cladding or waterproofing membrane. Because the layers are not bonded into a composite unit, the structural and thermal elements work independently rather than together.
How does thermal performance differ between warm and cold deck panels?
Warm deck panels deliver more predictable thermal performance because the insulation is continuous and fully bonded, eliminating cold bridges at joints and fixings. Cold deck structures rely on the ventilated cavity to manage heat and moisture movement, which means thermal performance depends heavily on how well the ventilation gap is detailed and maintained.
In a warm deck sandwich panel, the core material determines the thermal resistance. Closed-cell PP foam and PET foam cores resist heat transfer across the panel thickness. Because the core is bonded to both skins, there are no air pockets or gaps where thermal bridging can develop within the panel itself. The weak points are at panel edges and fixings, which is why edge sealing matters.
Cold deck structures use the ventilated air gap to carry away moisture-laden air before it can condense on the structural deck. This works well when the ventilation path is unobstructed, but it introduces variability. If the gap becomes blocked or the insulation layer is poorly detailed at junctions, thermal performance drops and condensation risk rises. Warm deck panels avoid this variability by design.
Which core structure offers better structural strength?
Warm deck sandwich panels offer higher structural efficiency because the bonded core and skins act as a single composite unit, distributing loads across the full panel cross-section. Cold deck assemblies, where layers are not bonded together, cannot achieve the same stiffness-to-weight ratio because the layers carry loads independently rather than compositely.
The structural advantage of a warm deck panel comes from the sandwich effect: the face skins carry tensile and compressive forces, while the core resists shear. This is the same principle that makes an I-beam efficient. A PP honeycomb core at 140 kg/m³, for example, delivers high compressive and shear resistance while keeping the panel light. A 30 mm PP sandwich panel floor is self-supporting with no aluminium subframe required, which removes a production step for truck body builders and reduces overall assembly cost.
Cold deck assemblies can carry significant loads through the structural deck layer, but the insulation and cladding above it contribute little to structural performance. For applications where the panel itself must span unsupported, carry point loads, or resist impact, a bonded warm deck sandwich panel is the stronger choice.
When should you choose a warm deck over a cold deck structure?
Choose a warm deck sandwich panel when you need a single element to deliver structural performance, thermal resistance, and low weight simultaneously. Cold deck assemblies suit applications where the structural deck and insulation can be treated as separate systems and where ventilation of the cavity is practical to detail and maintain.
For commercial vehicle bodies and transport applications, truck floors, trailer walls, and scaffolding boards, warm deck sandwich panels are the standard choice. The panel must carry loads, resist impact, and keep weight low to maximise payload. In 2026, EV fleet procurement adds a further argument: lighter body structures extend battery range in electric trucks and vans. Every kilogram of structural dead weight reduces range per charge, making weight reduction a range argument, not just a payload argument. This is a standard procurement question from EV fleet buyers.
Cold deck construction is more common in building envelopes where the structural deck is fixed to a frame and the insulation and cladding above it can be detailed separately. If your application allows this separation and you need the ventilation cavity to manage moisture in a static structure, cold deck detailing can work well. If you need a panel that travels, flexes, or must be as light as possible, the warm deck composite approach is more practical.
How does moisture and condensation behave in each core type?
In a warm deck sandwich panel with a closed-cell core, moisture cannot enter the core because the cell structure does not absorb water. Condensation risk is managed by the panel’s thermal mass and the continuity of the insulation layer. In a cold deck structure, the ventilated cavity is the primary moisture management tool, carrying humid air away before it reaches the dew point on the structural deck.
Closed-cell PP foam and PP honeycomb cores used in warm deck panels have very low water absorption. Panels tested to ISO 62 show less than 1.5% water absorption across the product range, which means the core does not act as a moisture reservoir even in wet operating environments. For truck bodies and scaffolding boards that are regularly washed down or exposed to rain, this is a practical advantage.
The risk point in a warm deck panel is at the edges. If the core is exposed at cut edges, moisture can wick in over time. Edge sealing and edge bending protect the core from UV and moisture ingress, which is relevant for scaffolding boards and truck floors that face repeated washing and outdoor exposure. A cold deck structure manages moisture differently: the cavity must be ventilated to the outside, and if that ventilation path is blocked, condensation accumulates on the structural deck, which can cause long-term deterioration.
What are the weight differences between warm and cold deck panels?
Warm deck sandwich panels are significantly lighter than equivalent cold deck assemblies for the same structural and thermal performance. A bonded sandwich panel achieves its stiffness through composite action, so less material is needed overall. A cold deck assembly stacks separate structural and insulation layers, which adds weight without the structural benefit of composite bonding.
To put this in practical terms: a 17 mm glass-fibre-reinforced PP foam wall panel has a total areal weight of around 3,638 g/m² based on Compoform’s own testing. A cold deck assembly delivering similar thermal resistance would typically require a structural deck plus separate insulation boards, with combined areal weights considerably higher. The difference becomes commercially significant when weight directly affects payload or range.
For trailer and truck body manufacturers, every kilogram saved in the body structure is a kilogram that can carry revenue-generating cargo. For EV operators, the same kilogram saved translates directly into extended range per charge. Warm deck sandwich panels consistently outperform cold deck assemblies on this metric because the composite structure does more with less material.
How Compoform Helps with Sandwich Panel Core Selection
We manufacture thermoplastic sandwich panels with PP honeycomb and PET foam cores for commercial vehicle bodies and scaffolding. When you are deciding between core configurations, panel thickness, and skin layup, we work through the structural and thermal requirements of your specific application before recommending a specification.
Here is what that looks like in practice:
- Application review: We assess your load case, span, operating environment, and assembly process to identify the right core density and skin configuration for your panel.
- Custom dimensions: Panels are available in custom dimensions cut to your exact specification, including large-format panels up to 13,500 mm × 2,950 mm, so you are not forced to work around standard sizes.
- Edge protection guidance: We advise on edge sealing and edge bending to protect the core from moisture and UV in demanding environments such as scaffolding boards and truck floors.
- Lifespan argument: Thermoplastic composite panels last three to four times longer than plywood in heavy transport applications, based on industry experience. Over a 15-year vehicle life, that eliminates two to three replacement cycles. Once you account for panel cost, installation labour, and vehicle downtime, the composite is cheaper per year of service even at a higher unit price.
- EV procurement support: If your customers are transitioning to electric vehicles, we can help you quantify the weight saving and translate it into a range argument for their procurement teams.
We do not ship panels to spec and move on. We review your design, flag integration issues before production starts, and stay involved through the build process. If you are specifying a sandwich panel for a truck body floor, trailer wall, or scaffolding board and want to work through the core options with an application engineer, get in touch with our team to start the conversation.