Metal Sandwich Panel Selection Guide: Cores, U-Values and Climates

The building envelope is where a steel structure earns or loses its operational performance. Between 60 and 70% of the heating and cooling load of an industrial or commercial building flows through the roof and walls, which means the cladding specification — not the frame — usually decides the energy bill. Metal sandwich panels dominate the market precisely because they combine structure, insulation and finish in one fast-to-install component: two profiled steel faces bonded to an insulation core, spanning between purlins or girts without secondary framing.

But "sandwich panel" hides four product families with very different physics. Choose EPS where fire is a marginal concern and budget dominates; choose PIR where energy performance governs; choose rock wool where fire and acoustic performance are non-negotiable; choose the right thickness for the climate or pay for it every month for 40 years. This guide compares the core materials with real numbers — thermal conductivity, fire class, density — explains the U-value calculation, covers the two details that cause most field failures (wind uplift fastening and cold bridging), and closes with a climate-based configuration table you can take to a tender.

1. Core Materials Compared

Four cores dominate panel production worldwide. Their differences are summarised below with representative values from Chinese and European production practice; specific products carry their own test certificates, and the specification should always cite the tested value, not the catalogue band.

Core material	Density (kg/m³)	Thermal conductivity λ (W/mK)	Fire behaviour (approx. Euroclass / GB 8624)
EPS (expandable polystyrene)	12-20	0.035-0.041	E (FR-treated: E-B, s1-d0) / GB 8624 B1-B2
XPS (extruded polystyrene)	25-45	0.028-0.034	E (FR: C-s2,d0 variant) / GB 8624 B1-B2
PIR / PUR (polyisocyanurate / polyurethane)	38-45	0.022-0.028	B-s2,d0 (PIR) to C / GB 8624 B1
Rock wool (mineral fibre)	80-150	0.036-0.045	A1-A2 / GB 8624 A (non-combustible)

The engineering reading of this table: PIR delivers the best insulation per millimetre (about 50-60% higher R-value per thickness than rock wool), EPS/XPS are the low-cost options with modest fire performance, and rock wool is the only non-combustible core — which is why it dominates warehouses, logistics centres, food plants and any building where fire compartments and evacuation are safety-critical. XPS additionally offers near-zero water absorption and higher compressive strength, which matters for cold-room floors and high-load walls; EPS is the most moisture-sensitive and degrades in performance if the skins are punctured.

2. Thermal Performance: From Lambda to U-Value

The heat transmittance U-value of a panel follows the series-resistance model: U = 1 / (R_si + R_panel + R_se), where R = t / λ for each layer and the internal/external surface resistances are typically 0.10 and 0.04 m²K/W. The steel faces contribute negligibly. Example: a 100 mm PIR panel at λ = 0.024 W/mK gives R = 4.17, so U ≈ 1 / (0.14 + 4.17) ≈ 0.23 W/m²K. The same thickness in rock wool at λ = 0.040 gives U ≈ 0.32, and EPS at 0.038 gives U ≈ 0.31.

Three consequences follow. First, for a given U-value target, PIR needs the thinnest panel — 100 mm PIR equals roughly 140-150 mm rock wool — which changes wall depth, girt spacing and handling weight. Second, the λ value in the catalogue is a fresh, dry core value; EPS and to a lesser extent PUR pick up moisture over decades, and the design U-value should be based on the declared long-term thermal resistance (the PIR industry’s declared value over 25 years is roughly 5-10% below the fresh value; EPS can degrade more where the envelope leaks). Third, U-value alone does not control condensation: the dew-point position inside the panel must be checked for the project’s interior conditions, which is where vapour-barrier detailing — not just insulation math — comes in (Section 5).

3. Fire Performance and Building Code Drivers

Fire is usually the binding constraint on core selection, and the numbers are unambiguous:

• Rock wool is non-combustible (Euroclass A1/A2, GB 8624 A): it does not contribute fuel, does not release toxic smoke in quantity, and maintains compartment integrity during a fire. Mandatory or strongly preferred for high-occupancy, logistics and food-industry buildings in most codes.
• PIR (Euroclass B-s2,d0 with standard facing) is combustible but self-extinguishing and low-smoke; it is the default for general industrial and commercial envelopes where fire loads are moderate, and most codes accept it with sprinklers or for buildings of limited height.
• EPS/XPS (Euroclass E to B depending on flame-retardant treatment) melt, drip and contribute fuel; their use is increasingly restricted by local codes for walls and roofs of public and high-risk buildings, though they remain legal and economical for low-risk single-storey sheds in many jurisdictions.

The specification should cite the whole-panel fire test (e.g. the large-scale test under EN 14509 / GB/T 23932 for the panel system), not just the core board class — facings, adhesives and edge seals change real behaviour. And when fire-rated walls or roofs are required (1-4 h integrity), rock wool panels with tested fire rating, plus intumescent or gypsum protection at structural members, are the only route.

4. Wind Uplift Resistance and Fastening

Roof panels are lifted, not pressed. Under negative (suction) pressure in a windstorm, the panel must transfer load to the purlins through its fasteners — and the failure mode is fastener pull-through or panel-fixing-strip detachment, not panel crushing. The governing numbers come from wind tunnel codes (EN 1991-1-4 / GB 50009): the roof is divided into zones with net pressure coefficients C_p,net that rise from interior zones to edge and corner zones by factors of roughly 1.5-2.5. A flat roof corner zone in high wind can see suction exceeding 2.0 kPa even at 50-year return winds — per square metre of panel, that is 200 kg of lift to hold down.

Three specification rules manage this:

• Fastener density follows the wind zones, not a single pattern. Interior fastening at, say, 500 mm spacing typically tightens to 250-300 mm in edge zones and 150-250 mm at corners, decided by the manufacturer’s tested pull-through capacity for the panel and purlin gauge.
• Through-fixed screws vs hidden-fix (clip) systems. Through-fixed is strongest and simplest but creates point thermal bridges (Section 5); clip systems with standing-seam-style profiles reduce bridges and suits conditioned buildings, with tested uplift capacity per clip from the manufacturer’s data sheet.
• Test evidence over catalogue claims. Wind uplift performance per EN 14509 Annex E (or FM 4471 for FM-required projects) should be a contractual deliverable, including the fastener schedule and purlin compatibility. On high-exposure sites, ask for the negative-pressure test result for the actual panel span, not a generic brochure figure.

5. Cold Bridges, Condensation and Detailing

Most envelope performance complaints — condensation drips, black staining at the purlin line, energy bills above calculation — trace to cold bridging and joint detailing rather than the core. The main bridges in a panel envelope:

• Through-fix screws: every screw is a steel bridge from the outer face to the inner face. Insulated washer assemblies and spacer blocks under the fixing reduce the bridge; hidden-fix clip systems eliminate the screw thermal path through the insulation.
• Purlin and girt contact: the panel underside touches the cold steel of the purlin flange. Thermal spacer strips (e.g. 50 mm polyamide or foam pads) plus sealing tape at the purlin face break the bridge and the air path.
• Joints and penetrations: panel butt joints must carry factory-applied seals or foam injection; every cut panel (roof lights, equipment curbs) is a potential vapour path if the inner liner is not re-sealed. Cut edges need protection to prevent face corrosion at the friction-cut zone.
• Vapour control: in cold climates, the vapour retarder side (inner face) must have lower permeance than the outer face so moisture moves out, not in; in hot-humid tropics the logic inverts and the design must be checked accordingly. A dew-point calculation with the actual winter interior conditions should be part of the envelope design, not an afterthought.

6. Climate-Based Configuration Recommendations

The table below gives practical starting configurations; final values depend on the local energy code, exposure and building use. Thickness and U-value targets reference current Chinese and international energy efficiency practice.

Climate zone (examples)	Recommended core	Typical thickness and U-value	Key requirements
Mild temperate (central-south China, Mediterranean)	EPS or PIR	75-100 mm; U 0.35-0.45 W/m²K	Standard detailing; condensation check in winter; EPS FR where fire code allows
Hot and humid (tropical, south China)	PIR (or XPS where moisture-critical)	60-80 mm; U 0.30-0.40 W/m²K	Vapour control reversed (vapour retarder at exterior side); solar reflectance topcoat; sealed joints against humidity ingress
Cold and continental (north China, central Asia)	PIR (energy) or rock wool (fire)	120-150 mm; U ≤ 0.24 W/m²K	Through-fix bridges eliminated or heavily mitigated; vapour barrier at warm side; frost-proof seals; thermal spacer at purlins
Industrial / high fire risk (warehouses, logistics, food)	Rock wool	100-150 mm; U 0.25-0.35 W/m²K	Non-combustible core class A; whole-panel fire test documentation; tested wall/roof details at junctions

Key Takeaways

• Core choice is a fire-and-energy decision, not a price decision: PIR λ 0.022-0.028 W/mK gives the thinnest, highest-performance envelope; rock wool (A class) is the only non-combustible route; EPS/XPS are budget options with real fire and moisture limitations.
• U-value math is transparent: U = 1/(0.14 + t/λ), so 100 mm PIR ≈ 0.23 W/m²K while 100 mm rock wool ≈ 0.32 — and condensation, not just insulation, must be checked by dew-point analysis.
• Wind zones drive fastening: corner and edge zones carry 1.5-2.5× the suction of the interior, and fastener spacing follows tested pull-through capacity per EN 14509 Annex E or FM 4471.
• The envelope fails at its details: through-fix screws and purlin contact are the classic cold bridges; thermal spacers, hidden-fix clips and vapour control at the correct side prevent the classic complaints.
• Configure by climate and fire class: 75-100 mm EPS/PIR for mild zones, 60-80 mm PIR with inverted vapour logic for tropics, 120-150 mm PIR or rock wool at U ≤ 0.24 for cold climates, and rock wool for high-fire-risk uses.

We specify, supply and install sandwich panel envelopes as part of complete steel building systems — see our prefabricated building systems and metal roof envelope engineering ranges for the integration of panels with the primary structure. If you are selecting panels for a specific site and need the U-value, wind zone and fire class worked out, contact our engineering team with the location, building use and local code — you will receive a panel configuration and cost comparison within 24 hours.