Sandwich panel is one of those products where the wrong choice is expensive twice. Once when you buy it. Again when the roof leaks, the cold room cannot hold temperature, or the fire inspector fails the building.

The core material inside the panel decides everything. Thermal performance, fire rating, moisture resistance, price. And most buyers pick wrong because they compare only the panel price per square meter without thinking about what the building actually needs.

This guide breaks down the three core types, how thick the panel should be, and what to check before you order from a supplier.

The Three Core Types: EPS, PU/PIR, and Rockwool

The short version:

• Budget warehouse with no fire code requirement? EPS is fine and cheap.
• Cold room or freezer? PU or PIR. The closed-cell structure keeps moisture out and insulation in.
• Fire safety is non-negotiable, like a garment factory or hospital? Rockwool, full stop.

Step 1: Match the Core to the Application

Step 2: Choose the Right Thickness

Panel thickness is the second most common mistake. Too thin and the building cannot hold temperature. Too thick and you are paying for insulation you do not need.

Decision rule: If the room needs to stay below 10°C, use at least 100 mm PU core. For freezer rooms below 0°C, 150 mm minimum. For anything below -25°C, 200 mm.

Always ask for the actual U-value certificate from the manufacturer, not just the thickness. A well-sealed 100 mm panel beats a poorly joined 150 mm panel every time.

Step 3: Check the Steel Facing

The outer skin of the panel matters almost as much as the core.

• Steel thickness: 0.3 mm is budget grade. For roofing and exterior walls, 0.4-0.5 mm is the sensible minimum. Heavier gauge resists wind and snow loads.
• Coating: PPGI (zinc) for inland, PPGL (galvalume) for coastal or humid areas. For high UV regions, PVDF coating is the standard for a 20+ year color life.
• Panel width: Standard 1000 mm or 1150 mm effective widths. Stick to standard sizes unless your project truly needs custom width, it costs less and installs faster.

Step 4: Connection System Matters

This is often overlooked but it determines air tightness and how fast the building goes up.

• Tongue and groove. Panels interlock, requires foam sealant at joints. Economical but slower to install.
• Cam lock. Steel cams embedded in the panel edges. No external fasteners, very tight joints, fast assembly. Preferred for clean rooms.
• Built-in gasket. EPDM rubber gasket along the panel perimeter. Best thermal break, prevents condensation. Found on premium panels.

For cold storage, the joint quality is what actually keeps the room airtight. A cheap panel with poor joints will frost up at the seams no matter how thick the core is.

Step 5: Verify the Supplier

• Core density test report. EPS minimum 15 kg/m³, PU minimum 35-45 kg/m³. Low-density core is a classic way to cut cost and performance.
• Fire rating certificate. For rockwool panels, confirm Class A1/A2 certification. For PU, check whether it is standard or PIR (PIR has better fire performance).
• Steel facing certificate. Confirm the PPGI/PPGL grade and coating weight on the MTC.
• Bonding quality. The core must be fully bonded to the steel facing. Check for delamination by tapping a sample panel.
• Third-party inspection. Will they allow SGS or BV at the factory?

Watch out for the low-density core trick. A 100 mm PU panel with 28 kg/m³ core costs less to make but insulates noticeably worse and loses strength. Always demand the density on the certificate.