Insulated sandwich panels

Definition and function

An insulated sandwich panel (also called an insulated metal panel, IMP) consists of two metal skins bonded to a rigid insulating core. The composite acts structurally: the skins carry bending stresses and the core carries shear, so panels can span between purlins or girts on their own. In one installation step the panel provides the weather skin, thermal insulation, vapour control and a finished interior surface.

Sandwich panels are used where thermal performance, hygiene or speed of enclosure matter – cold storage and food processing, cleanrooms, conditioned factories, offices and showrooms.

Role in the envelope and load path

Wind, snow and roof loads are carried by panel bending between supports, then through concealed or through-fixed fasteners to the secondary framing. Panel span tables depend on core type, thickness, skin thickness and temperature difference across the panel – a large difference (for example in freezers or with dark colours in sun) causes thermal bowing and additional stress. Panels are usually not relied on to brace the frame unless specifically designed.

Core types and typical sizes

• Panel thickness: 50–100 mm for walls and roofs of conditioned buildings; 100–200 mm for cold stores and freezers.
• Skins: metallic-coated, pre-painted steel about 0.40–0.60 mm, with food-safe or hygienic coatings for food plants.
• Cover width: commonly 1,000–1,200 mm; roof panels have a trapezoidal outer skin, wall panels flat, micro-ribbed or lightly profiled.

Factory-foamed versus field-assembled panels. Two very different products are both called "sandwich" or insulated panels. A factory-foamed (composite) panel is made on a continuous line where liquid PIR or PUR foam expands between the skins and bonds to them, or where mineral wool lamellas are glued in; the result is a single structural element. A field-assembled (built-up) system consists of an outer profiled sheet, insulation and a liner sheet installed separately on site, often with spacers or sub-girts.

Composite panels are tested and declared to EN 14509 in Europe, which covers mechanical properties, durability, thermal performance and reaction to fire of self-supporting double-skin panels.

Design considerations

• Fire: core type drives code compliance. Examples are EN 13501-1 classification and EN 14509 testing (Europe), FM 4880/4881/4471 approvals and NFPA 285 for exterior walls (US, where applicable), and AS 5637.1 and NCC requirements (Australia). Insurers often have their own requirements.
• Thermal and vapour performance: panel joints must be sealed on the warm side; cold rooms need vapour-tight joints, thermal breaks at floors and heated floors under freezers.
• Spans and wind: use product load tables with the correct temperature difference; fastener pull-through governs under suction.
• Thermal bowing with dark colours or large internal–external temperature differences.
• Detailing at openings, corners and penetrations to maintain the thermal and vapour envelope.

Supply and installation

World Steel supplies insulated sandwich panels together with matching trims and fasteners as part of complete building packages, with secondary framing laid out to suit panel spans. Installation:

1. Check framing line and plumb within the panel manufacturer's tolerances.
2. Handle panels with vacuum lifters or soft slings to avoid dents and delamination.
3. Engage side joints fully and fix with the specified fasteners and washers.
4. Seal joints and install trims, then remove protective film promptly – film left in the sun becomes difficult to remove.

Common mistakes and quality checks

• Selecting a core that does not meet the local fire code or insurer requirements.
• Joints not fully engaged, causing air leaks and condensation.
• Cut edges left unsealed, exposing the core to moisture.
• Protective film left on too long.
• Damage from storage without cover or on uneven ground.