How thick should sandwich panels be in a motorhome build?

compoform · 05 Sep 2026

For most motorhome builds, sandwich panel thickness ranges from 20 mm to 35 mm for walls, 25 mm to 40 mm for floors, and 18 mm to 30 mm for roofs, with the right choice depending on structural load, insulation requirements, and weight targets. Thermoplastic composite sandwich panels deliver far better stiffness-to-weight ratios than timber or metal at equivalent thicknesses, which is why they have become the standard choice for modern motorhome construction. The questions below unpack the specific decisions that determine the right specification for each zone of a build.

What factors determine the right panel thickness for a motorhome?

The right sandwich panel thickness for a motorhome is determined by four factors: the structural load the panel must carry, the span between supports, the thermal performance required, and the total weight budget for the vehicle. No single thickness suits every position in a build; each zone has a different combination of demands.

Structural load is the most immediate driver. Floor panels carry point loads from furniture, equipment, and occupants, so they need greater depth to resist deflection across unsupported spans. Wall panels carry lower direct loads but must resist wind pressure, racking forces during travel, and the weight of any mounted fixtures. Roof panels sit somewhere between the two, with lower point loads but large unsupported spans that require meaningful bending stiffness.

Span length matters as much as load. A panel that performs well over a 600 mm span may deflect unacceptably over 1,200 mm at the same thickness. Increasing core depth is the most efficient way to raise bending stiffness without adding significant weight, because the outer skins move further apart and the moment of inertia increases with the cube of the depth.

Thermal performance adds a third dimension. Thicker cores provide better insulation, but the relationship is not linear; core density and material type affect thermal conductivity as much as thickness does. Finally, every millimetre of additional thickness adds weight, which directly affects fuel consumption, payload capacity, and in 2026, battery range in electric motorhome platforms.

How thick should motorhome floor, wall, and roof panels be?

As a practical guide, motorhome floor panels typically run 25 mm to 40 mm, wall panels 20 mm to 35 mm, and roof panels 18 mm to 30 mm. These ranges reflect the different load and span conditions in each zone, not arbitrary convention.

Floor panels

Floors carry the highest concentrated loads in a motorhome; furniture legs, appliance bases, and occupant foot traffic all create point loads that the panel must distribute without deflecting. A 30 mm panel with a high-density PP honeycomb core is a common specification for motorhome floors, providing the stiffness needed to eliminate the need for a separate aluminium subframe beneath the floor. Removing that subframe saves both weight and a production step. For longer unsupported spans or heavier equipment layouts, 35 mm to 40 mm may be appropriate.

Wall panels

Walls in a motorhome build need to resist racking and wind loads during travel, support mounted fixtures, and contribute to the vehicle’s thermal envelope. A 20 mm to 25 mm panel with a PP foam core at 60 kg/m³ handles these demands well in most configurations. Where thermal performance is a priority, for example, in four-season motorhome and specialist vehicle applications, specifying a lower-density core at greater thickness can improve insulation without a proportional weight penalty.

Roof panels

Roofs span large areas with relatively low point loads, but they must resist deflection under their own weight and any accumulated load (such as a roof box or solar panels). An 18 mm to 25 mm panel is common, with 30 mm used where solar mounting or roof-rack loads are part of the design brief. Skin architecture matters here too; a cross-ply laminate distributes load more evenly across a wide span than a unidirectional skin.

Does a thicker sandwich panel always mean better insulation?

No. A thicker sandwich panel does not automatically deliver better insulation. Thermal performance depends primarily on the core material’s conductivity and density, not just its depth. Adding thickness improves insulation only when the core material itself has low thermal conductivity; increasing the depth of a dense, conductive core adds weight without proportional thermal benefit.

PP foam cores at lower densities (40 to 60 kg/m³) offer better insulation than higher-density cores at the same thickness, because more of the cross-section is air-filled closed-cell foam rather than solid polymer. PP honeycomb cores, which are used where structural performance is the priority, have a different thermal profile because the hollow tube geometry creates air columns rather than a continuous foam matrix.

In practice, the best insulation outcome for a motorhome panel comes from selecting a core material with inherently low conductivity and then specifying the minimum thickness that meets the structural requirement. Chasing insulation performance purely through thickness tends to produce panels that are heavier than necessary, a real cost in a vehicle where every kilogram of structural weight reduces payload or range.

How does panel thickness affect the weight of a motorhome build?

Panel thickness directly affects weight, but the relationship depends on core density. A thicker panel with a low-density core can weigh less than a thinner panel with a high-density core. The key metric to track is areal weight, grams per square metre, rather than thickness alone.

For a motorhome with roughly 30 m² of total panel area (floor, walls, and roof combined), a difference of 500 g/m² in areal weight translates to 15 kg across the full build. That is not a trivial number. In a vehicle already carrying water tanks, batteries, furniture, and occupants, 15 kg of unnecessary structural weight either reduces payload or, in an electric platform, shortens range per charge.

This is a standard procurement question from EV motorhome buyers in 2026. Lighter body structures extend battery range directly; every kilogram of structural dead weight reduces range per charge. Weight reduction in composite panels is therefore a range argument as much as a payload argument, and EV fleet buyers increasingly evaluate panel specifications on both dimensions simultaneously.

Thermoplastic sandwich panels consistently outperform timber and metal on this metric. A 25 mm composite floor panel is substantially lighter than a plywood equivalent of the same thickness while delivering greater stiffness and a much longer service life, eliminating the replacement cycles that add cost over a vehicle’s operating life.

What skin and core combinations work best at different thicknesses?

The most effective skin and core combinations depend on whether the priority is structural performance, thermal performance, or surface finish, and these priorities shift with thickness. At thinner specifications, skin architecture carries more of the structural load; at greater thicknesses, core stiffness becomes the dominant factor.

Thin panels (15 mm to 20 mm)

At this range, a cross-ply glass-fibre skin over a 60 kg/m³ PP foam core works well for wall cladding where loads are modest and weight is a priority. The cross-ply orientation distributes stress in both directions, which suits panels that span in two axes. A PET-film surface finish provides a clean, paintable face without adding significant weight.

Mid-range panels (20 mm to 30 mm)

This is the most versatile range for motorhome applications. A 4-ply glass-fibre skin over a PP honeycomb core at 140 kg/m³ delivers the stiffness needed for floor applications, while a 2-ply skin over a lower-density PP foam core suits walls and roofs where thermal performance matters more than maximum load capacity. Anti-skid surface finishes are available for floor panels where slip resistance is part of the specification.

Thicker panels (30 mm to 40 mm)

At 30 mm and above, PP honeycomb cores at 140 kg/m³ provide the structural depth needed for heavily loaded floors or long unsupported spans. The bending stiffness at this thickness is substantially higher than at 15 mm, making these panels suitable for self-supporting floor structures that eliminate the need for a separate subframe. Skin ply count can be adjusted to balance weight against the specific load requirement.

When should you specify custom panel dimensions instead of standard sizes?

You should specify custom panel dimensions whenever standard sizes create unnecessary joints, waste material through cutting, or fail to match the structural geometry of your build. For motorhome construction, custom dimensions are often the more practical choice, not the premium option, because they reduce assembly complexity and improve structural continuity.

Standard panel sizes are optimised for general availability, not for the specific span lengths, corner radii, or opening positions in a given motorhome design. When a standard panel requires multiple cuts and joints to fit a wall section, each joint becomes a potential weak point and adds assembly time. A single custom-dimensioned panel that covers the full wall section in one piece eliminates both problems.

Custom dimensions also allow you to match panel thickness to the actual load and span in each zone of the build, rather than defaulting to a single thickness across the whole vehicle. This zone-specific approach produces a lighter overall structure than specifying the heaviest panel everywhere to cover the worst-case load.

Panels are available in dimensions up to 13,500 mm × 2,950 mm, which covers the full length of most motorhome bodies in a single piece. At that scale, the case for custom specification over standard sizes is straightforward.

How Compoform Helps with Motorhome Panel Specification

We work with motorhome builders and coachbuilders at the design stage to match panel configuration to the actual structural and thermal demands of each zone in the build. That means reviewing your floor plan, span lengths, load points, and weight targets before recommending a specification, not shipping a standard panel and leaving integration to you.

What we bring to a motorhome panel project:

Zone-specific configuration: We specify core material, core density, skin ply count, and surface finish independently for floor, wall, and roof panels, so each panel is optimised for its actual role, not over-engineered across the board.
Custom dimensions up to 13,500 mm × 2,950 mm: Panels cut to your exact design geometry, reducing joints, simplifying assembly, and improving structural continuity.
Self-supporting floor panels: A 30 mm PP honeycomb floor panel is stiff enough to eliminate the aluminium subframe in many motorhome floor designs, removing a production step and reducing total build weight.
Thermoplastic cores only: We work with PP honeycomb and PP foam cores, both of which are fully recyclable and compliant with EU ELV Directive requirements, relevant for OEMs managing end-of-life obligations.
Long service life: Thermoplastic composite floors last significantly longer than plywood in comparable service conditions, eliminating replacement cycles over the vehicle’s operating life and reducing total cost of ownership.

If you are specifying panels for a new motorhome platform or re-evaluating an existing design, bring us in at the drawing stage. We review your design, flag configuration issues before production, and optimise the panel specification for your assembly process, not just the load requirement on paper. Talk to our engineering team about your project.