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In short
- PU insulates well and costs the least, but it is combustible and melts and drips in a fire. Suited to general use where fire risk is low.
- PIR insulates best of the three (λ approx. 0.020–0.023 W/m·K) and is far harder to ignite than PU. It is the default core for cold rooms, cleanrooms and factory walls.
- Rockwool has a non-combustible core and is the choice for fire walls and acoustic walls. A 100 mm panel held integrity for 4 hours and insulation for 62 minutes under BS 476 Part 22. It is heavier and needs about 1.6× the thickness of PIR for the same insulation.
What is a sandwich panel (Isowall)?
An insulated sandwich panel is a three-layer prefabricated building element: an outer metal skin, an insulating core and an inner metal skin, bonded into one panel. The panel insulates and carries load on its own, and installs quickly on a steel frame with no masonry or plastering.
In Thailand the same product is called แซนวิชพาเนล (sandwich panel), insulated wall panel, cold room panel or Isowall, a name widely used locally for this type of panel. The European product standard for these panels is EN 14509 (Self-supporting double skin metal faced insulating panels).
The main uses are walls and ceilings of cold rooms and freezers, cleanroom walls in pharmaceutical, food and electronics plants, external walls and roofs of factories and warehouses, operating theatres, refrigerated truck bodies and prefabricated buildings.
Panel build-up and joints
Facings
Most panels use PPGI (pre-painted galvanised steel) or pre-painted aluminium-zinc (AZ) coated steel, approx. 0.4–0.6 mm thick. The standard colour for cleanrooms and cold rooms is off-white RAL 9002. Chemical or wash-down areas may need stainless steel or special coatings. Three surface profiles are common: Flat, Mesa (wide shallow ribs) and Micro (fine ribs). Ribs stiffen the skin and hide minor dents.
Core
PU and PIR cores are made by injecting liquid chemicals between the steel sheets; the foam expands and bonds directly to the skins. Most modern panels are made on a continuous automated line, which controls density and thickness more consistently than discontinuous (one-panel-at-a-time) production. Rockwool cores are cut into lamellas, laid with the fibres perpendicular to the skins and glued to the steel.
Joints
| Joint type | Description | Typical use |
|---|---|---|
| Slip joint | Tongue and groove slide together; fast to install, flush finish | Internal walls, cleanrooms, cold rooms |
| Secret joint (secret fix / hidden fix) | Fixing screws hidden inside the joint; no exposed screw heads; good weather tightness | External factory walls, high-bay warehouses |
| Frame joint / non-progressive | Steel or aluminium frame locks each panel; one panel can be removed without dismantling its neighbours | High-spec cleanrooms, rooms that may be re-planned |
| Cam-lock | Embedded hooks pull adjacent panels tightly together | Prefabricated cold rooms and freezers |
| Roof joint | Overlapping ribs with a joint cover over the screw heads | Factory and warehouse roofs |
PU core (polyurethane, PUR)
PU or PUR is rigid polyurethane foam made by reacting a polyol with an isocyanate (MDI). It is more than 90% closed-cell. It is the oldest sandwich panel core and is still common in general cold room work.
- Thermal conductivity (λ)
- Typically approx. 0.022–0.026 W/m·K (depends on blowing agent and foam age)
- Core density
- Typically 38–42 kg/m³
- Blowing agent
- Square Panel uses cyclopentane, which has zero ozone depletion potential, in place of HCFC-141b, which has been phased out under the Montreal Protocol
- Fire behaviour
- Combustible; softens, melts and drips at high temperature; needs fire retardants. Typically DIN 4102-1 class B2–B3
Advantages
- Lowest cost per square metre
- Good insulation and light weight
- Low water uptake thanks to closed cells
- Widely available and familiar to installers
Disadvantages
- Ignites and spreads flame more easily than PIR
- Heavy smoke; melts and drips in a fire
- Often not accepted by insurers or FM-specified projects
- Lower continuous service temperature than PIR
Example: a 50 mm Square Panel PU panel achieved Class 1 surface spread of flame under BS 476 Part 7 (2020).
PIR core (polyisocyanurate)
PIR is polyisocyanurate foam. It uses the same raw-material family as PU but with much more MDI (an isocyanate index typically above 200, versus about 100–130 for PU) plus a trimerisation catalyst. The molecules form heat-stable isocyanurate rings. In a fire the foam surface forms a char layer that protects the foam behind it and helps the flame die out quickly.
PU and PIR look the same: both are yellow foams. The only reliable way to tell them apart is the manufacturer's fire test reports and certifications.
- Thermal conductivity (λ)
- 0.023 W/m·K (ASTM C518, SPS data sheet)
0.020 W/m·K at −10 °C mean temperature (EN 12667, independent laboratory, 2025) - Core density
- 40 kg/m³ (DIN EN ISO 845) · overall density 42 kg/m³
- Compressive strength
- 1.4 kgf/cm² perpendicular · 3.18 kgf/cm² parallel (ISO 844)
- Dimensional stability
- Within ±3% at −30 °C, 70 °C and 70 °C/95% RH (DIN EN 1604)
- Reaction to fire
- EN 13501-1: B-s1, d0 (50 mm panel) · DIN 4102-1: B2 · ASTM E84 (foam core): FSI 15, SDI 90 = Class A
- Approvals
- FM Approved to FM 4880, 4881 and 4882 (SPS PIR Firewall, Class 1, no height restriction)
Why two λ values for PIR?
Foam conductivity depends on the test temperature: the colder, the lower. 0.020 was measured at a −10 °C mean temperature, close to cold room conditions; 0.023 was measured under standard ASTM C518 conditions. When comparing brands, check that λ was measured to the same standard and temperature.
Foam conductivity depends on the test temperature: the colder, the lower. 0.020 was measured at a −10 °C mean temperature, close to cold room conditions; 0.023 was measured under standard ASTM C518 conditions. When comparing brands, check that λ was measured to the same standard and temperature.
Advantages
- Best insulation, so thinner panels reach the same U-value
- Hard to ignite, low smoke, no flaming droplets (B-s1, d0 achievable)
- Can achieve FM Approval, which insurers recognise
- Light weight similar to PU; low water uptake
Disadvantages
- Costs more than PU
- Still a combustible core, not a non-combustible material
- Not a fire-resistance-rated wall unless the full wall system is tested
- Slightly more friable than PU
Rockwool core (stone wool / mineral wool)
Rockwool, also called stone wool or mineral wool, is made by melting volcanic rock and spinning it into fibres. It is inorganic and non-combustible. Rockwool sandwich panels are the main choice for fire walls, fire compartment walls and acoustic walls in factories.
- Thermal conductivity (λ)
- 0.036 W/m·K (ASTM C518, SQUAREROCK™) · typically 0.036–0.044
- Core density
- 125 kg/m³ (SPF-100 fire test panel) · 0.5 mm steel facings both sides
- Melting point of fibres
- Above 1,000 °C (ASTM E794)
- Load capacity
- 150 kg/m² at 2.5 m span (EN 14509) · compressive strength 6,000 kg/m² (ASTM C365)
- Moisture
- Water vapour sorption < 0.04% by volume (ASTM C1104) · water absorption < 1.0 kg/m², partial immersion (EN 1609)
- Fire resistance
- BS 476 Part 20 & 22 (non-loadbearing wall, 3 × 3 m), see table below
- Thicknesses
- 50, 75, 100 mm · secret, slip and frame joints
| Criterion | Time achieved | Meaning |
|---|---|---|
| Integrity | 240 min (4:00 h) | No sustained flaming on the unexposed side and no gap passing the 6 mm gauge for the full 4 hours, when the test was stopped |
| Insulation | 62 min (1:02 h) | Maximum temperature rise on the unexposed face stayed within 180 °C |
The specimen was a 3 × 3 m wall of 100 mm panels with a 125 kg/m³ stone wool core and 0.5 mm steel on both faces. The furnace followed the BS 476 standard curve to about 1,150 °C at 4 hours. The wall therefore met 60 minutes on both integrity and insulation, and integrity alone for 4 hours.
| Thickness | 125 Hz | 250 Hz | 500 Hz | 1000 Hz | 2000 Hz | 4000 Hz |
|---|---|---|---|---|---|---|
| 50 mm | 0.30 | 0.85 | 1.00 | 1.00 | 0.96 | 0.84 |
| 100 mm | 0.70 | 1.00 | 1.00 | 1.00 | 0.99 | 0.89 |
Advantages
- Non-combustible core: no smoke or flaming droplets from the core
- Suitable for fire walls: a 100 mm panel met 60 min fire resistance and 4 h integrity (BS 476 Part 22)
- Best sound absorption and insulation of the three
- Dimensionally stable at high temperature; resists insects and mould
Disadvantages
- Higher λ than PIR: needs about 1.6× the thickness for the same insulation
- Heavier: the core is about 3× as dense as foam (125 vs 40 kg/m³)
- Panel edges and joints must be sealed to keep moisture out
- Costs more for the same U-value; not used for freezer rooms
PU vs PIR vs Rockwool: comparison table
| Property | PU | PIR | Rockwool |
|---|---|---|---|
| Thermal conductivity λ (W/m·K) | 0.022–0.026 | 0.020–0.023 (SPS) | 0.036 (SPS) |
| Thickness for U ≈ 0.46 W/m²·K | approx. 48–56 mm | 50 mm | approx. 78 mm |
| Core density (kg/m³) | 38–42 | 40 (SPS) | 125 (SPS) |
| Reaction to fire | Combustible, melts and drips | Hard to ignite, chars · B-s1, d0 (SPS) | Non-combustible core (A1 material) |
| Fire-resistance-rated wall | Not suitable | Not suitable unless system-tested | Suitable · 100 mm: integrity 240 min, insulation 62 min (BS 476-22) |
| Smoke in a fire | High | Low (s1) | Virtually none from the core |
| Water uptake | Low (closed cell) | Low (closed cell) | Edges must be sealed |
| Sound | Poor | Poor | Good |
| Panel weight | Light | Light | Heavy |
| Relative cost | Lowest | Medium | Highest |
Choosing thickness: U-value
The U-value (heat transfer coefficient) is the heat in watts that passes through 1 m² of wall for each degree of temperature difference, in W/m²·K. The lower the U-value, the better the insulation. A quick estimate is λ divided by insulation thickness in metres.
Recommended PIR panel thickness for cold rooms
| Thickness (mm) | U-value (W/m²·K) | Suggested room temperature |
|---|---|---|
| 42 | 0.54 | Air-conditioned room |
| 50 | 0.46 | 0 °C and above |
| 75 | 0.31 | −10 to 0 °C |
| 100 | 0.23 | −20 to −10 °C |
| 125 | 0.18 | −30 to −20 °C |
| 150 | 0.15 | −40 to −30 °C |
| 200 | 0.12 | −50 to −40 °C |
Fire standards: reading test results correctly
Fire test results for sandwich panels cover two different questions. Mixing them up is the most common reason the wrong wall gets specified.
- Reaction to fire: how easily the material ignites, how fast flame spreads, and whether it produces smoke and droplets. PIR performs well here.
- Fire resistance: how many minutes the whole wall stops fire and heat reaching the other side. This is the job of a fire wall, where Rockwool performs well.
| Standard | What it measures | Result scale |
|---|---|---|
| EN 13501-1 | Reaction to fire (uses EN 13823 SBI and EN ISO 11925-2 results) | A1, A2 (non/limited combustibility) → B, C, D, E, F · smoke s1–s3 · droplets d0–d2 |
| EN 13501-2 / EN 1364-1 | Fire resistance of walls | EI 30, EI 60, EI 120 … (E = integrity, I = insulation, number = minutes) |
| BS 476 Part 6 / Part 7 | Surface spread of flame | Part 7: Class 1 (best) to Class 4 |
| BS 476 Part 20 / Part 22 | Fire resistance of non-loadbearing walls | Minutes, reported separately for integrity and insulation |
| DIN 4102-1 | Combustibility of building materials (Germany) | A1, A2 (non-combustible) · B1 (low flammability) · B2 (normal) · B3 (highly flammable) |
| ASTM E84 / UL 723 | Surface burning characteristics (USA) | Class A: FSI 0–25 · Class B: 26–75 · Class C: 76–200 (SDI ≤ 450) |
| FM 4880 / 4881 / 4882 | Large-scale fire tests (room test, 16 ft parallel panel) by FM Approvals | Class 1 · 4881 external walls (incl. wind and hail) · 4882 smoke-sensitive occupancies such as cleanrooms, pharmaceutical and food areas |
| EN 14509 | Product standard for complete sandwich panels (strength, durability, thermal) | Basis for CE marking in Europe |
Note: a fire test result applies only to the specimen and thickness tested. When comparing, ask for the full report and check thickness, core type and test date. A “fire wall” claim should always be backed by a fire resistance result in minutes.
Choosing the core by wall application
| Application | PU | PIR | Rockwool | Main reason |
|---|---|---|---|---|
| Cold room / freezer | ◐ | ● | ○ | Needs low λ and moisture resistance; PIR adds fire safety |
| Cleanroom, pharma, food plant | ◐ | ● | ◐ | Smoke-sensitive rooms often specify FM 4882 panels; Rockwool where fire separation is required |
| Operating room / hospital | ◐ | ● | ◐ | Flush, easy-clean surfaces and low smoke |
| Factory / warehouse external wall | ◐ | ● | ◐ | Solar heat control, wind load (FM 4881), insurer acceptance |
| Fire wall / fire compartment | ○ | ○ | ● | Needs a fire resistance rating in minutes and a non-combustible core |
| Acoustic wall, machine or generator room | ○ | ◐ | ● | Stone wool absorbs and blocks sound better than foam |
| Factory roof | ◐ | ● | ◐ | Light weight reduces roof structure |
| Temporary building / tight budget | ● | ◐ | ○ | Low fire risk and lowest cost |
Products by application: Cold room panel · Clean room panel · Fire proof panel · Acoustic panel · PIR roof panel
What drives sandwich panel price
Prices per square metre for PIR, PU and Rockwool panels move with steel and chemical prices, so this page gives no figures. The biggest price factors are:
- Core type: PU is usually cheapest, then PIR, then Rockwool
- Thickness: more insulation costs more
- Steel facing: thickness (e.g. 0.4 or 0.5 mm) and type (PPGI, AZ, stainless)
- Paint system: e.g. anti-fungal or chemical-resistant coatings
- Joints and accessories: flashings, sealant, doors, windows, ceiling suspension
- Required approvals: e.g. FM Approval or a fire resistance rating
- Quantity, delivery distance and installation
The most accurate way to price a project is to send the drawings or room sizes, operating temperature and fire requirements to the manufacturer.
How to choose a sandwich panel manufacturer
Many buyers search for a “sandwich panel manufacturer in Thailand” without knowing what to compare. Use this checklist for any supplier.
- Own factory, and clarity on whether production is continuous or discontinuous
- Core type stated clearly as PU or PIR, not just “foam”
- Full fire test reports from independent laboratories, matching the thickness offered
- Verifiable certification such as ISO 9001 or a listing in the FM Approval Guide (approvalguide.com)
- A data sheet giving λ with its test standard, density and U-value by thickness
- Design and installation team, or installation instructions that match the tested system
- Reference projects of the same type: cold rooms, cleanrooms or factories
Frequently asked questions
Is a sandwich panel the same as Isowall?
Yes, they are the same type of product: a prefabricated insulated wall panel with steel skins on both sides and an insulating core. “Isowall” is a name widely used in Thailand; the international term is insulated sandwich panel.
What is the difference between PU and PIR? Can you tell by looking?
PIR uses much more MDI and forms isocyanurate rings, so it resists heat and fire better than PU and insulates slightly better. You cannot tell them apart by eye; both are yellow foams. Ask for fire test results such as EN 13501-1 or FM Approval.
Which core insulates best?
PIR insulates best (λ approx. 0.020–0.023 W/m·K), followed by PU (0.022–0.026) and Rockwool (approx. 0.036). At the same thickness a PIR panel has the lowest U-value.
Is a PIR panel non-combustible?
No. PIR is a combustible core, but it is hard to ignite, produces little smoke and does not drip. Good PIR panels reach EN 13501-1 B-s1, d0 and FM Approval Class 1. For a non-combustible core, use Rockwool.
Should a fire wall use Rockwool or PIR?
Fire walls and fire compartment walls should use Rockwool sandwich panels with a fire resistance result in minutes, for example to BS 476 Part 22 or EN 1364-1. PIR suits general walls where reduced flame spread is required.
How long can a Rockwool panel wall resist fire?
It depends on thickness and each manufacturer's test. Look at both criteria: integrity and insulation. For example, a 100 mm Rockwool sandwich panel wall with a 125 kg/m³ core tested to BS 476 Part 22 in 2023 held integrity for 4 hours and insulation for 62 minutes, meeting 60 minutes on both criteria.
What panel thickness for a −20 °C cold room?
Based on the PIR guide table, rooms from −20 to −10 °C use about 100 mm; below −20 °C use 125 mm or more. The final thickness depends on room size and refrigeration load and should be calculated by an engineer.
What insulation is used for cleanrooms?
PIR is the usual choice because it is light, insulates well and produces little smoke. Smoke-sensitive rooms such as pharmaceutical and food plants often specify FM 4882 panels. Walls that must separate fire compartments use Rockwool. Slip joints and removable frame joints are the common systems.
What is the difference between 50 mm and 100 mm Rockwool panels?
A 100 mm panel insulates about twice as well (U approx. 0.36 vs 0.72 W/m²·K), absorbs low-frequency sound better (125 Hz: 0.70 vs 0.30) and generally resists fire longer. The fire resistance time must come from a test report for that thickness; for example, a 100 mm panel tested to BS 476 Part 22 held integrity for 240 minutes and insulation for 62 minutes.
What is a secret joint panel?
A secret joint (secret fix) hides the fixing screws inside the panel joint, so no screw heads show on the outside. The wall looks clean and the screws are protected from water. It is common on external factory and warehouse walls.
What is EN 14509?
EN 14509 is the European product standard for self-supporting double-skin metal-faced sandwich panels. It covers strength, durability and thermal testing and is the basis for CE marking.
What is the difference between BS 476 Part 7 and Part 22?
Part 7 measures surface spread of flame and gives Class 1–4. Part 22 measures how many minutes the complete wall resists fire. A Class 1 result to Part 7 does not make a panel a fire wall.
Why does FM Approval matter?
FM Approvals is the testing arm of FM, an industrial property insurer. Panels approved to FM 4880 are fire-tested at room scale, so many insurers and multinational projects require them. Listings can be checked in the FM Approval Guide.
How is a PIR roof panel different from metal sheet roofing?
Metal sheet is a single steel layer that lets a lot of solar heat through. A PIR roof panel has insulation between two steel layers, so it greatly reduces heat gain and rain noise and needs no separate under-roof insulation.
What information do I need to buy sandwich panels?
Room sizes or building drawings, operating temperature, type of use (cold room, cleanroom, external wall), fire or insurance requirements, colour and facing type, and the installation site.
Glossary
- λ (lambda)
- Thermal conductivity of a material in W/m·K. Lower is better insulation.
- U-value
- Rate of heat transfer through a wall in W/m²·K. Lower is better insulation.
- Closed cell
- Foam whose cells are not interconnected, so it absorbs little water.
- PPGI
- Pre-painted galvanised iron (steel).
- AZ
- Steel coated with an aluminium-zinc alloy for better corrosion resistance.
- Reaction to fire
- How easily a material ignites and spreads flame.
- Fire resistance
- How many minutes a wall stops fire passing through it.
- B-s1, d0
- EN 13501-1 class: limited contribution to fire (B), very low smoke (s1), no flaming droplets (d0).
- EI 60
- A wall that maintains integrity (E) and insulation (I) for 60 minutes.
- Lamella
- Stone wool cut into strips and laid with fibres perpendicular to the skins to increase panel strength.
- Non-progressive
- A wall system in which any single panel can be removed without dismantling its neighbours.
About the author
This guide was prepared by Square Panel System Co., Ltd. (บริษัท สแควร์ พาแนล ซิสเต็ม จำกัด), a Thai company that designs, manufactures and installs insulated sandwich panels. Founded in 2000. Head office in Bangna, Bangkok; factory in Si Mahosot District, Prachinburi. ISO 9001:2015 certified, with PIR panels made on a continuous automated line.
- SPS PIR Firewall PIR panels, 42–200 mm, FM Approved to FM 4880, 4881 and 4882
- SQUAREROCK™ Rockwool-core panels, 50, 75 and 100 mm, fire-tested to BS 476 Part 22
- Cold room, cleanroom, operating room and acoustic walls, and Square Roof roof panels
PIR interior panels Properties of PIR FM Approved panels squarepanel.com Request a quotation
Tel. +66 2 744 6300 · Line: @squarepanel · Email: [email protected]
References
- Square Panel System, data sheets “SPS PIR Firewall” and “SQUAREROCK™ Panel” (2026)
- FM Approvals, Approval Report Project No. PR464932, SPS PIR Firewall, Class 4880/4881/4882 (31 July 2025)
- Fire Safety Research Center (FSRC), Faculty of Engineering, Chulalongkorn University, Test Report FSRC-044/66: Fire resistance of non-loadbearing elements of construction, Sandwich Panel Model SPF-100, BS 476 Part 20 & 22:1987 (21 December 2023)
- Thermal conductivity test to EN 12667:2001, PIR foam core 40 kg/m³ (December 2025), and ASTM E84-24 (February 2025), independent laboratory
- EN 13501-1:2018 Fire classification of construction products and building elements
- EN 14509:2013 Self-supporting double skin metal faced insulating panels
- BS 476 Parts 7, 20 and 22 · DIN 4102-1 · ASTM E84 / UL 723 · FM Approval Standards 4880, 4881, 4882
