Sandwich panels (insulated composite panels): core types, advantages, and the fire risks you need to know

Sandwich panels are a fast, attractive option for annexes and warehouses, but the difference between one panel and another is not just price — it is the core type and how it behaves in fire.

1) What is it?

An insulating material between two thin facings, usually metal (steel, aluminium) or plastic (PVC, fibreglass), typically used for walls and sometimes for ceilings, floors and roofs1. Core types include1:

• Mineral fibre (rock wool or glass fibre): non-combustible.
• PIR (polyisocyanurate).
• Phenolic foam (PF).
• PUR (polyurethane).
• Polystyrene (EPS or XPS).

2) Why are they so widespread?

The insurer RSA summarises their popularity1: good thermal and acoustic insulation, tolerance of harsh conditions (frequent cleaning, moisture, low temperatures), ease of cleaning, good mechanical resistance, light weight, easy and fast installation, and lower cost than other solutions. So they are used in food processing, cold storage and clean rooms, and as cladding on many building types.

3) Fire: the key point

The same guide explains1:

• Combustible insulation significantly increases the extent of a fire if it becomes involved.
• EPS melts and forms a flammable liquid on the floor of burning buildings.
• Fire can spread rapidly inside the panels, shielded from extinguishing water, and sprinklers and extinguishing systems cannot control a fire in the core — so fires often end in total loss.
• Burning plastic insulation gives off dense, highly toxic black smoke.

RSA notes that class A is only likely to apply to mineral fibre panels, that the best expected for plastic-insulated panels is class B, and that the best way to reduce the hazard is to use non-combustible panels, or at least "approved" panels from bodies that test at large scale such as FM, UL and LPCB1.

4) Practical precautions from the same guide

1. Record the core type and panel locations in the building on a drawing1.
2. No welding, and no cutting with tools that make sparks or heat (torches, high-speed saws) on the panels; use low-speed reciprocating saws and drills1.
3. Keep electrical equipment away from combustible panels: e.g. 50 mm for switches and sockets, 200 mm for electrical cabinets and cable trays; suspend lights rather than recessing them into panels1.
4. Seal penetrations with non-combustible material, not foam1.
5. Inspect panels regularly and cover any exposed core with fixed steel plating1.

5) Before buying

Practical questions based on the above.

• Exactly what core? Ask for the data sheet and fire classification certificate.
• For residential use or near heat sources: prefer rock wool.
• Check civil defence and municipal requirements for the building type and use.
• Ask about core thickness and insulation value, and how joints are sealed against leaks.

6) Pros and cons