Is polyurethane (PIR) flammable?

PIR is thermoset - it doesn't melt or drip. It chars on the surface creating a protective crust, but produces toxic smoke (hydrogen cyanide) if it burns.

What's the difference between PUR and PIR?

PUR (polyurethane) burns more easily with a toxic flame. PIR (polyisocyanurate) chars and self-extinguishes, offering much better fire protection.

PIR & Polyurethane: The Truth About Foams and Their Behaviour at High Temperatures

If you want the ultimate thermal insulation with the smallest possible thickness, polyurethane foams are the undisputed kings. Yet, regarding fire safety, there is enormous confusion.

To clear the landscape, we must first understand that not all "yellow foams" are the same. There are two basic categories: Polyurethane (PUR) and its "evolved" sibling, Polyisocyanurate (PIR).

Both are plastics. However, unlike polystyrene (EPS) which is thermoplastic (it melts with heat), PUR and PIR foams are thermoset. What does that mean in practice when fire breaks out?

1. The "It Doesn't Burn" Myth and the Truth of "It Chars"

The biggest lie circulating is that polyurethane and PIR "don't burn." Of course they burn - they're organic materials (Euroclass B, C or D)! They simply burn differently.

PIR foam: the outer surface chars, forming a hard black crust

🔥 Polystyrene (EPS)

When a flame strikes a polystyrene board, it melts instantly, disappears and drips liquid, flaming plastic.

🛡️ PIR (Polyisocyanurate)

When a flame strikes a PIR board, the material doesn't melt and doesn't drip (index d0). Instead, its outer surface burns and creates a thick, black, hard carbon crust (the Charring phenomenon).

✅ The Crust-Shield

That carbon crust acts as a shield! It prevents oxygen from reaching the foam's interior and delays further combustion. If you remove the flame, fire on PIR usually extinguishes itself (self-extinguishing).

2. The "Dark Side": Toxicity and Smoke

If PIR doesn't melt and resists fire with its crust, why isn't it considered the perfect material? Because chemistry always has a price.

Toxic smoke from polyurethane combustion - hydrogen cyanide and carbon monoxide

☠️ Lethal Smoke

When PUR and PIR foams burn, they may not drip, but they produce lethal smoke. Besides the well-known carbon monoxide, polyurethane combustion releases Hydrogen Cyanide, one of the most toxic and rapidly lethal gases that exist.

⚠️ Real Tragedies

In several tragedies (such as nightclubs with exposed polyurethane foam soundproofing), the victims didn't burn from the flames, but collapsed within minutes from inhaling hydrogen cyanide.

3. The Golden Rule of Application

So is PIR dangerous? No, if applied correctly. PIR and PUR are excellent and safe materials, as long as they are never exposed (bare) indoors.

Correct application of PIR: sandwich panels with steel sheeting, or behind plasterboard

🏗️ Industrial Sandwich Panels

In industrial roof panels (sandwich), the PIR foam is trapped between two thick steel sheets. The steel protects the foam from direct contact with the flame.

🏠 Residential Buildings

In homes, if you install polyurethane boards on the walls or ceiling, you MUST mandatorily enclose them behind fire-rated plasterboard or thick render.

⚠️ Rule: No polyurethane foam (PUR or PIR) should ever remain exposed in an interior space. It becomes a source of toxic smoke if it catches fire.

The Experiment in Our Model (The Roof Renovation in the 4×4)

Experiment: bare PUR foam in a short circuit vs certified PIR sandwich panel

We're building our roof and we want top-tier thermal insulation.

❌ Scenario A (The Old Polyurethane PUR)

We spray bare polyurethane foam under the roof tiles and leave it uncovered. A short circuit throws sparks. The foam ignites. It produces a dense, yellow-black, poisonous smoke that fills the house in 3 minutes.

✅ Scenario B (The Modern PIR Panel)

We install certified sandwich panels (steel – PIR – steel). Fire breaks out in the room. The flame hits the ceiling's steel sheet. The sheet heats up, the PIR behind it "chars" locally, but refuses to go up in flames and releases no smoke into the room (since it's protected by the metal). We have time to extinguish the fire. The panel was damaged, but it didn't turn the roof into an inferno.

Final Takeaway: The "yellow foam" PIR is a huge technological leap over polystyrene regarding fire reaction (it doesn't melt, it doesn't drip). However, it remains a plastic that produces toxic smoke if burned. Use it confidently for its magical insulating properties, but always make sure to "dress" it properly, keeping it away from fire.

Insulation Combustibility Guide: Euroclass A1 to F Crash Test: EPS (Polystyrene) vs Rock Wool in Fire Euroclasses (A1-F): Material Reaction-to-Fire Categories Fire Protection in External Insulation (ETICS): How to Prevent the Building Envelope Becoming a Torch Relay

Fire Protection: Passive Safety & Building Protection

Return to category.

Go to category

Central Guide

Return to the central guide.

Go to guide