Fibre Reinforced Facade Encyclopedia: Composition
A sandwich panel is a composite element made of two thin, stiff facing skins bonded to a thick, light core. The skins carry the bending stresses as tension and compression, the core carries shear and, more importantly, holds the skins apart. The result is a panel with the bending stiffness of a much thicker solid section at a fraction of the weight, which is why the arrangement is used for everything from aircraft floors to insulated wall panels to large fibre reinforced polymer facade elements.
Separating two 2 mm skins with a 40 mm core produces roughly 400 times the bending stiffness of the same two skins laid together, for an added weight of a few kilograms per square metre. Bending stiffness rises with the square of the distance between the skins.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Skin thickness | 2 to 5 mm per face | FRP laminate; thinner for metal faced panels |
| Core depth | 10 to 100 mm | Set by span and deflection limit, not by strength |
| PET foam core density | 80 to 200 kg/m³ | Recyclable, good at elevated temperature |
| PU and PIR foam core density | 35 to 60 kg/m³ | Lightest and cheapest; combustible |
| Aluminium honeycomb core density | 50 to 100 kg/m³ | Highest stiffness to weight; poor at point loads |
| Mineral wool core density | 100 to 150 kg/m³ | The non-combustible core option; heaviest |
| Completed panel weight | 10 to 35 kg/m² | Typical for an FRP skinned architectural panel |
| Governing check | Deflection, then core shear | Strength is rarely the limit in a facade panel |
| Fire | A property of the tested assembly only | Core, skins, adhesive and joints all count |
Why the geometry works
Bending stiffness of a section is proportional to the second moment of area, which grows with the square of the distance of material from the neutral axis. Moving the skins apart therefore buys stiffness far faster than thickening them. A sandwich is an I-beam flattened out: the skins are the flanges, the core is the web, and the core only has to be strong enough in shear and compression to keep the two flanges working together.
The consequence for design is that a sandwich panel is usually governed by deflection and by core shear, not by skin strength. Increasing core depth solves deflection cheaply; increasing core density solves shear. Adding skin thickness is the least efficient response to either.
Core materials
- Closed cell PET foam: 80 to 200 kg/m³, recyclable, stable to around 100 °C, the current default for architectural FRP
- Polyurethane and polyisocyanurate foam: lightest and cheapest, good insulation, combustible and limited above 80 °C
- Extruded and expanded polystyrene: low cost insulation cores, combustible, poor with styrene based resins
- Aluminium and aramid honeycomb: highest stiffness per kilogram, needs care at fixings and edges
- End grain balsa: high compressive strength, natural material, sensitive to moisture ingress
- Mineral wool: the non-combustible option, heavy, low shear strength, used mainly in metal faced insulated panels
Core selection is a fire decision as much as a structural one. It also sets the fixing strategy, because most foam cores cannot take a load directly and need local inserts or hardwood or high density foam blocks laminated in where anchors land.
How sandwich panels fail
Six failure modes matter, and a design check should address each. Skin yielding or rupture in tension or compression. Skin wrinkling, where the compression skin buckles locally into the core, which is governed by core modulus rather than core strength. Core shear failure, usually a diagonal crack near a support. Core crushing under a concentrated load or at a fixing. Debonding at the skin to core interface. And global buckling where the panel is loaded in plane.
The fire question
A sandwich panel hides a large volume of core material inside the wall, which is exactly the geometry that caused the combustible cladding failures of the past two decades. A polymer core behind a thin skin can be ignited from a joint, a penetration or a damaged edge, and it then burns in a concealed cavity where it is difficult to reach.
Under NCC 2022 Volume One, the external walls of Type A and Type B construction must be non-combustible under the Deemed to Satisfy provisions, which a combustible core does not satisfy on its own (check the edition adopted in your jurisdiction). The alternative is a Performance Solution supported by a full scale facade test, AS 5113 in Australia or BS 8414 with BR 135 in the UK, and that test result applies to the assembly as built, including core, skins, adhesive, joints, cavity and fixings. The combustible cladding and aluminium composite panel entries cover the regulatory history.
Manufacture and specification
Sandwich panels are produced by bonding pre-made skins to a core, by laminating skins directly onto the core wet, or by vacuum infusion through a grooved and perforated core that lets resin travel. Infusion gives the most consistent bond and the highest fibre content; hand bonding is more forgiving of tolerance. Curved and double curved panels need a core supplied as scrim backed blocks that can be shaped to the mould.
Specify the skin laminate, the core type and density, the core depth, the bond method, the deflection limit under the design wind pressure, the fixing detail with its local reinforcement, and the fire evidence for the complete assembly. A specification that names only a panel thickness has specified almost nothing.
Standards and references
- ASTM C393 Core Shear Properties of Sandwich Constructions by Beam Flexure
- ASTM C297 Flatwise Tensile Strength of Sandwich Constructions
- ASTM D1781 Climbing Drum Peel for Adhesive Bonds
- ASTM C365 Flatwise Compressive Properties of Sandwich Cores
- AS 5113 and BS 8414 with BR 135 (full scale facade fire tests for assemblies)
- NCC 2022 Volume One, Specification 7 external wall requirements
- EN 14509 Self-supporting double skin metal faced insulating panels
Related terms
Questions specifiers ask
Why is a sandwich panel stiffer than a solid panel of the same weight?
Because bending stiffness depends on how far the material sits from the neutral axis, and it rises with the square of that distance. Moving two thin skins apart with a light core multiplies stiffness while adding very little mass.
Are sandwich panels combustible?
It depends entirely on the core, the skins and the adhesive. A mineral wool cored, metal faced panel can be tested as non-combustible; a polymer foam core behind a polymer skin cannot. The rating belongs to the tested assembly, named standard and testing body included.
What core should be used in an FRP facade panel?
Closed cell PET is the common choice for its stability, recyclability and bond performance. Honeycomb is used where weight is critical. Whichever is selected, the fire pathway for the completed wall has to be resolved before the core is fixed.
How are sandwich panels fixed?
Through local hard points laminated into the core at anchor positions, so that the fixing load goes into a solid insert rather than crushing the foam. Edge closures seal the core against water and are part of the fire detail as well.
What limits the span of a sandwich panel?
Usually deflection under wind load, then core shear near the supports. Both are solved most efficiently by increasing core depth rather than by thickening the skins.
Working out whether a facade element should be a sandwich, a ribbed single skin or a monocoque? Our engineering team can size it against your wind pressures. Talk to our engineering team
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.