Fibre Reinforced Facade Encyclopedia: Composition
A foam core is a low density layer bonded between two fibre reinforced polymer skins so that the skins sit apart and act like the flanges of an I beam. The skins carry bending in tension and compression; the core carries the shear between them and holds them at a constant separation. Because bending stiffness rises with roughly the cube of the panel depth, a thin core adds enormous stiffness for very little weight. It also introduces a combustible layer into the build-up, which governs where cored construction can be used on an external wall.
Separating two 2 mm laminate skins with a 20 mm core raises the bending stiffness of the panel by roughly a hundred times while adding about 2 kg per square metre.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Core density range | 40 to 250 kg/m³ | 60 to 120 kg/m³ covers most panel work |
| Core shear strength | 0.5 to 3.5 MPa | Rises with density; the governing property |
| Core shear modulus | 15 to 90 MPa | Sets panel deflection under distributed load |
| Compressive strength | 0.6 to 5 MPa | Matters at fixings and under clamping |
| Typical core thickness | 10 to 50 mm | Facade and interior panel range |
| Skin to core flatwise tensile | above 0.8 MPa typical requirement | Tested to ASTM C297 |
| Service temperature, crosslinked PVC | about 60 to 80 °C | PET and SAN higher; check the grade |
| Combustibility | All polymer foams are combustible | Not a non-combustible core |
How a cored panel works
A sandwich panel is an engineered beam turned into a sheet. The skins are stiff and thin and take the bending stresses; the core is weak and thick and takes the shear. Provided the bond between them holds, the assembly behaves as one section with a depth it could never achieve as a solid laminate of the same weight.
The design consequence is that the usual failure modes are not skin tensile failure. They are core shear failure, where the core tears diagonally between the skins; skin wrinkling, where the compression skin buckles locally into the core; indentation at a point load or fixing; and debonding at the skin to core interface. Each is checked separately, and core shear is normally the one that governs a wind loaded facade panel.
Core materials compared
- Crosslinked PVC: the default structural foam. Good stiffness to weight, good shear strength, reliable resin bond, closed cell so it does not take up water. Brittle under impact and limited to roughly 60 to 80 °C
- Linear PVC: tougher and far more ductile than crosslinked PVC, absorbing impact energy rather than cracking. Lower stiffness and shear modulus for the same density, and a lower service temperature
- SAN, styrene acrylonitrile: tougher than crosslinked PVC at comparable stiffness and with better elongation, at a higher price
- PET: thermoplastic, recyclable, thermoformable, good temperature resistance and low cost. Lower shear strength and shear modulus than PVC at the same density, so panels are thicker
- PU, polyurethane: cheap and an excellent insulator, but with poor shear properties, poor skin bond and friability at the cut face. It is an insulation core, not a structural one
- End grain balsa: not a foam at all, but the usual alternative. Very high compressive and shear stiffness for its density, and a natural material with lower embodied carbon, but it absorbs water readily and its properties vary between pieces
In architectural work the choice usually narrows to crosslinked PVC or PET. PET is gaining where recyclability and processing temperature matter; PVC still holds the advantage on shear modulus per unit density.
The skin to core bond
A sandwich panel is only as good as its bond line. If the skins debond from the core the section loses its depth instantly and the panel reverts to two thin, floppy laminates. Debonds are also hard to see: they show as a slight dimple or a dull sound under a tap test rather than as a crack.
Three things protect the bond. Surface preparation and resin uptake, which is why structural foams are supplied grooved, perforated or with a light scrim, giving the resin both a mechanical key and a path to flow. Correct resin selection, with vinyl ester resin and epoxy resin both bonding better to foam than a general purpose polyester resin. And process control: vacuum bagging or infusion pulls the skin down onto the core evenly, where hand lay-up over a core relies on roller pressure and can leave voids at the interface. Bond quality is verified by flatwise tensile testing to ASTM C297 and by climbing drum peel to ASTM D1781 on qualification samples.
The fire implication on a facade
Every polymer foam listed above is combustible. A cored panel therefore places a continuous layer of combustible material inside the external wall, concealed behind a laminate skin where it is not visible and cannot be inspected. This is the same geometry that made polyethylene cored aluminium composite panel a systemic failure: a thin non-visible core, continuous across the elevation, with cut edges exposed at joints.
Under the National Construction Code, external walls of Type A and Type B construction must be non-combustible under the Deemed-to-Satisfy provisions (check the edition adopted in your jurisdiction). No polymer foam core satisfies that. The alternative pathways are a Performance Solution supported by a full scale facade test to AS 5113 or BS 8414, in which the complete system including the core, the joints, the cavity and the cavity barriers is tested as it will be built, or the use of a genuinely non-combustible core such as mineral wool, which carries nothing like the same structural performance.
Where cored construction is avoided, stiffness has to come from geometry instead: folds, returns, integral ribs and the curvature of a monocoque panel. That is a design constraint with real consequences for panel size and flatness, and it is worth resolving early rather than at submission.
Specifying a core
State the core material and grade, the nominal density in kilograms per cubic metre, the thickness, the minimum shear strength and shear modulus, the maximum service temperature and the required skin to core flatwise tensile strength with its test method. Require the core manufacturer’s data sheet, not a generic foam description. For any external wall application, require the fire test evidence for the complete panel build-up rather than for the skins alone.
Standards and references
- ASTM C393 and ASTM D7250 (flexural properties and core shear of sandwich constructions)
- ASTM C273 (shear properties of sandwich core materials) and ASTM C365 (flatwise compressive strength)
- ASTM C297 (flatwise tensile strength of sandwich constructions) and ASTM D1781 (climbing drum peel)
- AS 1530.1 (combustibility) and NCC 2022 Volume One C2D10 (check the edition adopted in your jurisdiction)
- AS 5113 and BS 8414 parts 1 and 2 (full scale facade fire testing of the complete system)
- AS 5637.1 and EN 13501-1 (reaction to fire classification of the finished panel)
Related terms
Questions specifiers ask
Is a foam cored panel allowed on an external wall in Australia?
Not through the Deemed-to-Satisfy non-combustibility route, because polymer foams are combustible. It requires a Performance Solution supported by a full scale facade test to AS 5113 on the complete system as built, assessed by a fire engineer.
Which foam core is best for a composite panel?
Crosslinked PVC for shear stiffness per unit density, PET where recyclability and processing temperature matter, SAN where impact toughness governs, and end grain balsa where maximum stiffness and low embodied carbon outweigh moisture sensitivity. PU is an insulation core, not a structural one.
What usually fails first in a sandwich panel?
Core shear under distributed load, or debonding at the skin to core interface. Skin tensile failure is rarely the governing mode, which is why core shear strength and bond strength are the properties to specify and test.
Why is the core grooved or perforated?
To let resin flow through during infusion and to give the cured skin a mechanical key into the core. A plain skinned foam relies on adhesion to a smooth face alone and is more prone to debonding.
Can expanded polystyrene be used as a core?
Not with a polyester or vinyl ester laminate laid directly onto it, because styrene dissolves it. Beyond that, EPS has poor shear properties and a well documented fire record in facades, so it is not a structural or an external wall core.
Need panel stiffness without a combustible core? Our composite panels use moulded geometry, and the fire evidence for each system is published in full. See the fire performance evidence
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.