On a continuous panel line, the question PIR vs PUR usually arrives with a specification sheet: a façade project asks for a better fire class, the customer wants the same thickness and the same line speed, and the plant has to decide whether its rigid polyurethane system can stay or a polyisocyanurate system must replace it. Both foams start from the same raw material families, polyols and polymeric MDI, yet they behave differently in the fire test, on the cutting saw and in the cost calculation.
PUR is rigid polyurethane foam made at an isocyanate index of about 105-130. PIR is polyisocyanurate foam made with a large isocyanate excess, typically an index of 250 or more, and a trimerisation catalyst, so the surplus forms heat-stable isocyanurate rings. PIR chars instead of melting and suits panels and boards with fire requirements; PUR is easier to process, bonds well and costs less.
- Same raw material families: both use polyols and polymeric MDI; the ratio and the catalyst package make the difference.
- Index is the dividing line: roughly 105-130 for PUR, typically 250 or higher for PIR.
- Fire behaviour: PIR forms a protective char and keeps its shape at higher temperatures; the fire class still belongs to the tested product.
- Insulation value is close: both reach aged lambda values typically around 0.022-0.028 W/m·K.
- PIR asks more of the process: tighter temperature control, friability management and attention to facing adhesion.
01What PUR And PIR Are
PUR and PIR are both closed-cell rigid foams. The names describe the dominant chemical link in the polymer, not two unrelated materials.
- PUR (rigid polyurethane): the isocyanate is dosed to match the hydroxyl groups of the polyol blend, plus a small excess. Almost every NCO group ends up in a urethane link.
- PIR (polyisocyanurate): the isocyanate is dosed in large excess. Once the hydroxyl groups are used up, three NCO groups at a time join into an isocyanurate ring. Most PIR foams still contain urethane links; the term describes foams in which the isocyanurate structure dominates.
Both are produced by mixing a formulated polyol blend (the B side in Europe, often called the A side in North America) with polymeric MDI. For a closer look at the board product itself, Pluskim explains what a PIR insulation board is and how it is produced.
02The Chemistry Difference In Three Variables
Three formulation variables turn a PUR system into a PIR system. If one of them is missing, the result is neither a good PUR nor a real PIR.
1. Isocyanate Index
The isocyanate index compares the isocyanate actually used with the amount needed to react with every OH group and the water in the blend. Rigid PUR typically runs at about 105-130. PIR runs at about 250 and often higher, so a large share of the isocyanate has no hydroxyl partner and is available for trimerisation.
2. Trimerisation Catalyst
Excess isocyanate does not form rings by itself. PIR blends add a trimerisation catalyst, commonly a potassium carboxylate such as potassium octoate or acetate, alongside the amine catalysts that drive the urethane and blowing reactions. The balance between them shapes the reaction profile: an early urethane rise followed by a second, trimerisation stage.
3. Polyol Choice
PUR systems rely mostly on short, highly functional polyether polyols started on sucrose, sorbitol or amines. PIR systems lean on aromatic polyester polyols, based on phthalic anhydride or recycled PET, whose aromatic rings add to char formation and keep viscosity manageable at high index. Many commercial systems blend both families.
03Fire Behaviour
The main reason to choose PIR is its behaviour in a fire. The isocyanurate ring is thermally more stable than the urethane link, so when PIR foam is exposed to flame it forms a carbonaceous char at the surface. The char slows heat transfer into the core and limits the contribution of the foam to the fire. PUR foam softens and decomposes at lower temperatures and needs more flame retardant to reach comparable results.
Two points matter when reading a specification:
- The class belongs to the product: in Europe, reaction to fire is classified under EN 13501-1, and the result depends on facings, thickness, joints and end-use conditions as much as on the foam. A foam that passes in one panel build can fail in another.
- Smoke and droplets are part of the class: the classification also reports smoke production and flaming droplets, which is where formulation quality shows.
The European standards catalogue is available through CEN-CENELEC. For a wider view of how polyurethane-based fire-resistant systems are selected by sector, see a guide to fire-resistant insulation.
04Thermal Performance
On insulation value, PIR and PUR are closer than the fire discussion suggests. Both owe their low thermal conductivity to a fine, closed cell structure filled with a low-conductivity blowing gas such as pentane or an HFO.
- Typical aged lambda: about 0.022-0.028 W/m·K for both, depending on blowing agent, cell size, density and facings. Gas-tight metal facings keep values lower over time than permeable facings.
- Cell structure matters more than the label: a coarse or partly open cell structure raises lambda in either foam. Surfactant choice and mixing quality are the levers.
- High-temperature service: PIR keeps its dimensional stability at higher service temperatures, which matters for roof panels under sun and for hot pipes.
PU Europe, the association of European polyurethane insulation producers, publishes background on the energy efficiency of PU insulation.
05Processing Differences On A Continuous Line
Switching a continuous panel line from PUR to PIR is a process change, not only a recipe change. The plants that make the transition smoothly prepare for four things.
- Temperature control: trimerisation needs heat. Component, conveyor and facing temperatures must sit in a narrower window than for PUR, and a cold facing in winter shows up quickly as poor curing at the skin.
- Friability: fresh PIR foam is more friable, especially at the surface. That affects saw cut quality, dust and the handling of panels in the first hours.
- Facing adhesion: surface friability weakens the bond to steel facings. Many lines apply a primer or adhesion promoter to the facing before the foam is laid down.
- Curing and stacking: PIR reaches full strength later; stacking and cutting schedules may need adjusting until the foam has cured.
Systems formulated for continuous lines are matched to line speed, flow and reaction time as a whole, which is why buying a PIR system as a complete, tested package is safer than adjusting a PUR blend in-house by adding isocyanate.
06PIR Vs PUR At A Glance
| Property | PUR (rigid polyurethane) | PIR (polyisocyanurate) |
|---|---|---|
| Isocyanate index | about 105-130 | about 250 and higher |
| Dominant link | Urethane | Isocyanurate ring (plus urethane) |
| Key catalysts | Amine gelling and blowing catalysts | Amine catalysts plus trimerisation catalysts (potassium carboxylates) |
| Typical polyols | Sucrose, sorbitol and amine polyethers | Aromatic polyester polyols, often blended with polyethers |
| Reaction to fire | Needs more flame retardant; softens earlier | Forms char; generally better class at equal build |
| Aged lambda | about 0.022-0.028 W/m·K | about 0.022-0.028 W/m·K |
| Service temperature | Lower | Higher, better dimensional stability |
| Friability and adhesion | Low friability, bonds readily | More friable; primer often used on facings |
| Process window | Wider | Narrower, more temperature sensitive |
| Raw material cost | Lower | Higher (more isocyanate per m³) |
07Where Each Foam Fits
- Roof and wall sandwich panels with a fire requirement: PIR is the usual choice, particularly for façades and buildings where insurers set the bar.
- Insulation boards for flat roofs and walls: mostly PIR, often with foil or mineral facings.
- Cold room panels and doors: PUR is common where no demanding fire class applies; its adhesion and low-temperature dimensional stability are an advantage.
- Refrigerators and appliances: PUR, because the foam must flow through long cavities before it gels.
- Spray foam and pipe insulation: mostly PUR systems, with fire-resistant grades where the project requires them.
The range of panel, spray and pipe systems Pluskim supplies is listed at pluskim.com/en/products/construction-insulation.
08Decision Checklist
Before changing systems, answer these questions with the project and the supplier together:
- Which fire class is required, and for which build? Ask for the classification report that matches your facings and thickness.
- What service temperature will the panel see? Dark roof panels in summer favour PIR.
- Can the line hold the temperature window? Check facing pre-heating, component conditioning and winter operation.
- How will adhesion be secured? Agree on primer, facing coating and adhesion tests before the first series run.
- What is the cost per panel, not per kilogram? Include isocyanate consumption, scrap during the changeover and any primer.
- Who supports the trial? A supplier that runs machine trials before series production shortens the transition.
09Conclusion
PIR and PUR are not competitors so much as two settings of the same chemistry. PUR gives a wide process window, easy adhesion and lower cost; PIR adds char-forming fire behaviour and higher temperature resistance at the price of more isocyanate and tighter process control. The right choice follows from the fire class, the service temperature and what the line can hold, and the change is safest when the system is designed and trialled as a whole. Producers weighing the switch on a panel line can start by comparing the reaction profiles of Pluskim's continuous line systems with their current blend.
10Sources And Further Reading
- PU Europe: energy efficiency of polyurethane insulation
- PU Europe: construction products and regulation
- CEN-CENELEC: European standards (including EN 13501-1)
- ISOPA, European Diisocyanate & Polyol Producers Association: applications of polyurethanes
11Frequently Asked Questions
What Is The Difference Between PIR And PUR Insulation?
Both are rigid foams made from polyols and polymeric MDI. PUR (rigid polyurethane) is run close to stoichiometry, at an isocyanate index of roughly 105-130. PIR (polyisocyanurate) uses a large excess of isocyanate, typically an index of 250 or more, and a trimerisation catalyst, so the surplus forms heat-stable isocyanurate rings. PIR chars in a fire and keeps its shape at higher temperatures; PUR is easier to process and bonds more readily to facings.
Is PIR Better Than PUR For Fire Performance?
At the same thickness and construction, PIR generally performs better in reaction-to-fire tests because the isocyanurate structure forms a protective char instead of melting. The fire class, however, belongs to the tested product, not to the foam alone: facings, thickness, joints and the specific formulation all count. In Europe the classification is made under EN 13501-1, so always ask for the report that matches your panel configuration.
Which Has The Lower Thermal Conductivity, PIR Or PUR?
The difference is small. Both reach among the lowest lambda values of common insulation materials, with aged values typically around 0.022-0.028 W/m·K depending on blowing agent, cell size, facings and density. The insulation value depends more on a fine, closed cell structure and on keeping the blowing gas in the cells than on the PIR or PUR label itself.
Why Is PIR Foam More Brittle Than PUR?
The isocyanurate ring is a rigid, highly crosslinked structure, so PIR foam is more friable, especially at the surface and in the first hours after production. That shows up as dust when cutting and as weaker adhesion to metal facings. Formulators counter it with polyol choice, catalyst balance, process temperature and, on panel lines, a primer or adhesion promoter on the facing.
What Isocyanate Index Is Used For PIR Foam?
PIR systems typically run at an isocyanate index of about 250 and often higher, compared with roughly 105-130 for rigid PUR. The high index only builds isocyanurate rings if a trimerisation catalyst, commonly a potassium carboxylate, is present and the reaction reaches a sufficient temperature. Raising the index without the right catalyst package simply leaves unreacted isocyanate.
When Should I Choose PUR Instead Of PIR?
PUR remains the practical choice when no demanding fire class is specified and processing ease, adhesion or cost matter most, for example in refrigerators, many cold room panels, pipe insulation and spray applications. PIR earns its higher raw material cost where the building code, insurer or customer asks for better reaction to fire or higher service temperatures, typically in roof and wall panels and insulation boards.