Sandwich panel

A sandwich panel is a building panel made of two thin facings, usually metal sheets, with a layer of insulation as its core. A typical example in Introduction to Fire Safety Engineering is an insulated steel roof with 100 mm of polyurethane foam between two steel sheets of 0.5 mm. For such products, fire performance can be dominated by construction details rather than by the surface material. New Zealand guidance therefore names metal-faced sandwich panels among the products that are not suitable for small-scale testing in the cone calorimeter. Panels with a combustible core are instead tested as full assemblies, for example in a room test or in a large corner test.

The core can be a foamed plastic, a light material with many fine voids that heats quickly at its surface and is therefore easy to ignite. Polyurethanes do not break down below about 475 K, and when they do they release hydrogen cyanide, carbon monoxide and isocyanates. At the same time the core supplies most of the thermal insulation of the panel, which slows the passage of heat to the unexposed side.

Construction and materials

A sandwich panel combines a thin, stiff skin on each side with a thick, light core. In the large corner test used by Factory Mutual Research Corporation, the wall and ceiling materials tested are typically panels with a metal skin over an insulation core material. The first edition of the New Zealand verification method C/VM2 describes the same product as a metal-skin panel assembly with a combustible core material. Sandwich construction is not limited to metal skins. In one series of corner fire tests, an 88 mm sandwich composite consisted of 76 mm of balsa wood between two facings of 6 mm fire-retarded glass-fibre-reinforced vinyl ester.

Insulating cores such as the polyurethane foam of the book's example belong to the group that New Zealand regulation calls foamed plastics. C/VM2 (second edition, 2025) defines these as combustible polymeric materials of low density, typically below 100 kg/m³. Fine voids typically make up 90 to 98 % of their volume. The listed examples include expanded or extruded polystyrene, phenolic, polyurethane and polyethylene foams. The 2013 commentary also refers to Australian standards for rigid cellular polyurethane, polyisocyanurate and polystyrene sheets for thermal insulation. The general fire behaviour of such polymers is treated in the article on plastics in fire.

Polyurethanes are made by the condensation of polyisocyanates and polyols. Their main use is as flexible or rigid foams or as thermal insulation. Introduction to Fire Safety Engineering gives polyurethane foam a thermal conductivity of 0.034 W/(m·K), a density of 20 kg/m³ and a specific heat of 1,400 J/(kg·K). Steel has a conductivity of 45.8 W/(m·K) and a density of 7,850 kg/m³. With these values, a square metre of the book's roof example contains about 7.9 kg of steel and only 2.0 kg of foam.

Fire behaviour of the core

The fire hazard of a sandwich panel lies mainly in its core. A material with low thermal inertia heats up quickly at its surface, and light insulation materials are an example. If such a material is combustible, the fast rise in surface temperature can contribute to fast ignition. NUREG-1805 explains that good insulators of low density typically ignite more quickly than poor insulators of high density. It names this as the main reason why foamed plastics are so dangerous in fires. Calculated with the book's property table, the thermal inertia of polyurethane foam is about 31 J/(m²·K·s^1/2), against about 2,000 for concrete.

When the core heats up, it decomposes. As a class, polyurethanes do not break down below 475 K, which is about 200 °C. The production of hydrogen cyanide and carbon monoxide increases with the pyrolysis temperature. Other toxic products include nitrogen oxides, nitriles and isocyanates. A major breakdown step splits the bond between polyol and isocyanate. The isocyanate vaporises and condenses again as smoke, while the liquid polyol remains and decomposes further.

Burning foam also produces heat and soot. The Australian Fire Safety Verification Method Data Sheets list rigid polyurethane foam with a soot yield of 0.118 kg/kg and a chemical heat of combustion of 16.9 MJ/kg. Polystyrene foam has a soot yield of 0.194 kg/kg and 25.5 MJ/kg. These values come from bench-scale tests in well-ventilated conditions. Under the ventilation-controlled conditions of many fully developed fires, the soot yield rises substantially and the effective heat of combustion falls.

The metal facings change how this fuel takes part in a fire. Guidance on the New Zealand lining tests notes that the cone calorimeter is unsuitable when the fire performance of an assembly is dominated by construction details rather than by the flammability of the surface. It is also unsuitable when significant mechanical damage occurs at full scale but not in small horizontal samples. For such products the commentary requires the ISO 9705 room test or another large-scale test.

Heat transfer through a panel

The insulating core also determines how much heat a panel passes from a fire to the other side. Introduction to Fire Safety Engineering gives a steady-state relation for a wall or roof of several layers. The heat flow per square metre equals the difference between the air temperatures on both sides, divided by the sum of all thermal resistances. Each solid layer adds its thickness divided by its thermal conductivity. Each surface adds a surface resistance, which is the inverse of its convective heat transfer coefficient.

$${\dot{q}}^{″}=\frac{T_1-T_4}{R_{si}+\frac{L_1}{{\lambda }_1}+\frac{L_2}{{\lambda }_2}+\frac{L_3}{{\lambda }_3}+R_{se}}$$

Symbol	Description	Unit
${\dot{q}}^{″}$	heat flow per unit area (heat flux) through the panel	W/m2
$T_1$, $T_4$	air or smoke temperature on the heated side and on the cool side	K
$R_{si}$, $R_{se}$	surface resistance on the heated side and on the cool side	m2·K/W
$L_1$, $L_2$, $L_3$	thickness of facing, core and facing	m
${\lambda }_1$, ${\lambda }_2$, ${\lambda }_3$	thermal conductivity of each layer	W/(m·K)

The relation assumes a steady state, with constant temperatures on both sides. In a real fire the smoke temperature changes and the construction warms up gradually, so the heat flow in the first minutes can differ considerably. The result is therefore an estimate for prolonged exposure and an upper value rather than a prediction over time. For a fully developed fire the book gives guide values of 20 to 25 W/(m²·K) for the convective heat transfer coefficient. For a standard fire 25 W/(m²·K) is usually taken, which gives a surface resistance of 0.04 m²·K/W.

In the book's worked example, a smoke layer of 400 °C lies under the insulated steel roof and the outside air is 10 °C. Both surfaces have a resistance of 0.04 m²·K/W. The foam layer of 0.1 m adds 2.94 m²·K/W and each steel sheet adds only about 0.00001 m²·K/W. The heat flow is then about 129 W/m². The core supplies about 97 % of the total resistance. Without it, the two steel sheets would pass about 4,900 W/m², roughly 38 times as much.

The book also shows that the temperature drop over each layer is proportional to its share of the total resistance. Applied to the roof example, almost the whole drop of 390 K occurs inside the foam. In the steady state, the face of the core behind the hot steel sheet would therefore reach about 395 °C. That is well above the temperature of about 200 °C below which polyurethanes do not break down. A good insulation value thus keeps the cool side cool, but it also concentrates the heating in the combustible core on the fire side.

Fire testing

Small-scale tests can rank many lining materials, but sandwich panels need large-scale tests. In New Zealand, results from the ISO 5660 cone calorimeter have been correlated with the time to flashover in the ISO 9705 room test. The 2013 commentary names metal-faced sandwich panels and foil-faced combustible insulation as products for which this correlation is not suitable. In the ISO 9705 room corner test, the linings are exposed to a burner of 100 kW for 10 minutes and then 300 kW for a further 10 minutes. Flashover is taken as a heat release rate of 1.0 MW.

A test standard written specifically for these products is ISO 13784 Part 1 (2002). Its title describes reaction-to-fire tests for sandwich panel building systems, with a test method for small rooms. The first edition of C/VM2 required metal-skin panel assemblies with combustible cores to be assessed only with ISO 9705 or ISO 13784 Part 1. In general, a product must be tested in a configuration that resembles its actual end use as closely as possible.

Factory Mutual Research Corporation has used a 25-ft corner test since 1971. The test structure is 7.6 m high, 15.2 m long and 11.6 m wide. It is exposed to a growing fire of about 340 kg of oak pallets stacked 1.5 m high, with a peak heat release rate of about 3 MW. A panel fails if, within 15 minutes, fire spreads to the limits of the structure or flames extend outside it through the ceiling smoke layer. In the FMRC Class No. 4880 standard for insulated wall or wall and ceiling panels, this corner test has been replaced by bench-scale tests in the ASTM E2058 fire propagation apparatus. Materials with a convective flame spread parameter of up to 0.39 pass, values from 0.47 fail, and the range between is uncertain.

In Europe, the Single Burning Item test is a regulatory heat release rate test for building products. It uses two wall panels in a corner configuration without a ceiling. The way its results are classified is described in the article on the Euroclass system. Corner tests on sandwich construction can produce high heat fluxes. In open corner tests with combustible linings, including the balsa sandwich composite, heat fluxes to the ceiling as high as 130 kW/m² were measured.

Regulation in New Zealand

The New Zealand verification method gives the most detailed rules on panels in the source documents used here. Under the first edition of C/VM2, as explained in the 2013 commentary, linings are classified with Group Numbers from 1 (best) to 4 (worst). The classification is based on their measured time to flashover in the ISO 9705 test. A Group Number 3 material has a total heat release of no more than 1 MW after 2 minutes at 100 kW. A Group Number 4 material exceeds 1 MW within those 2 minutes. Group Number 4 includes exposed polyurethane foams, which are not acceptable as linings in occupied spaces.

Where foamed plastics or combustible insulation form part of an element, the completed assembly must achieve the required Group Number. The foam itself must also meet the flame propagation criteria of AS 1366. Without a protective lining, the foam tested alone must reach Group Number 3. Normally a protective barrier between the foam and the room side is needed. According to the commentary, fire-retardant grades of foamed plastics are already generally used in the construction industry.

C/VM2 (second edition, 2025) refers to the separate Building Product Specifications for Group Numbers, surface linings and cladding. References to test standards such as ISO 9705, ISO 13784 and BS EN 13501 have been removed from the verification method itself. Material Group Numbers apply to the exposed surface of the wall or ceiling lining. In C/VM2, a combustible material is one that is neither non-combustible nor of limited combustibility. The wider meaning of these classes is discussed under combustibility.

Panels used as external walls also fall under the rules for vertical fire spread, the subject of the article on facade fire. For multi-level buildings taller than 10 m, C/VM2 (second edition, 2025) uses a design fire that exposes the cladding to 50 kW/m² for 15 minutes for importance levels 2 and 3. For importance level 4 the exposure is 90 kW/m². The entire external wall cladding system must either consist of non-combustible or limited combustible materials, or meet the acceptance criteria of the Building Product Specifications. Cavity barriers are required at each floor level.

Limits of current knowledge

The source documents used for this article contain no systematic data on the fire behaviour of complete sandwich panels. They give no measured times to failure of joints or facings and no account of fire spread hidden inside the core. They also give no data on panel roofs that carry solar installations, which are treated under photovoltaic system fire. The steady-state heat transfer calculation describes only the insulation of an intact panel. It does not show when a facing deforms, opens at a joint or lets flames reach the core.

Large single-storey buildings raise a further question. In a distribution centre with high racking, fuel at height can ignite as a secondary fire once the smoke layer exceeds 200 °C. Most materials ignite at temperatures between about 200 °C and 500 °C. How a panel roof above such storage contributes to a warehouse fire cannot be quantified from the sources available here.

See also

• Reaction to fire
• Single Burning Item test
• Cone calorimeter
• Large-scale facade fire test
• Fire resistance test

References

• Australian Building Codes Board. (2020a). Data sheets: Fire Safety Verification Method (Handbook annex, Print version 1.1).
• Babrauskas, V. (2002b). Heat release rates. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-1–3-37). National Fire Protection Association; Society of Fire Protection Engineers.
• Beyler, C. L., & Hirschler, M. M. (2002). Thermal decomposition of polymers. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 1-110–1-131). National Fire Protection Association; Society of Fire Protection Engineers.
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• Ministry of Business, Innovation and Employment. (2013). Commentary for Building Code clauses C1–C6 and Verification Method C/VM2.
• Ministry of Business, Innovation and Employment. (2025b). Verification Method C/VM2: Framework for fire safety design (2nd ed.).
• Tewarson, A. (2002). Generation of heat and chemical compounds in fires. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-82–3-161). National Fire Protection Association; Society of Fire Protection Engineers.
• Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.