Your building's insulation saves energy, but in a fire it can become a massive problem. One of the most critical factors to know before insulating your home is the Combustibility (Reaction to Fire) of the insulation material.
This guide classifies popular insulation materials into three "at-a-glance" categories, using the Euroclasses, so you know exactly what's going on your walls.
These materials, if not properly protected, can become fuel in a fire. They are the materials that "feed" the fire instead of stopping it.
Highly flammable. Ignites easily, melts, drips burning plastic droplets (d2) that can start fires below, and produces dense, black, toxic smoke. Must always be enclosed behind thick render (ETICS system) or masonry.
Similar behaviour to EPS. Ignites, melts and produces toxic smoke. Primarily used in foundations and floors.
PUR burns with a toxic flame. PIR (improved) chars on the surface, slowing the flame, but is not non-combustible.
These materials play no part whatsoever in a fire. They don't ignite, don't produce smoke, don't drip droplets. Fire finds no "fuel" to burn on them. They act as thermal shields.
The "king" of non-combustibility. Withstands over 1,000 °C without change. Doesn't ignite, doesn't melt (class s1,d0 - zero smoke, zero droplets). Made from molten volcanic rock (basalt).
Also entirely non-combustible. Made from molten glass. Same fire reaction as rock wool (doesn't ignite, no droplets or smoke).
Inorganic material with zero organic "fuel" for fire. Used in the most demanding fire protection applications.
These materials are not entirely non-combustible, but they don't feed the fire on their own. If the external flame source is removed, they stop burning.
Improved polystyrene with added retardant. Self-extinguishing: remove the flame and it goes out. Minimal smoke, zero droplets.
Char without flame, minimal smoke.
We're insulating our room's façade. Fire breaks out inside the room and the flames come out of the broken window and lick the external insulation.
The façade has EPS (class E) with a thin 3 mm render (poor workmanship). Flames emerge, crack the render and reach the polystyrene. In 8 minutes, the polystyrene melts, burning droplets drip onto the pavement and the entire façade is engulfed in flames (like the Grenfell Tower in London - a tragedy of 72 deaths in similar conditions).
The façade has rock wool (A1). Flames come out of the window and "lick" the insulation. The rock wool glows red but does not ignite. It drips nothing, produces no toxic smoke. Fire finds no fuel to spread to the upper floor. After 90 minutes, the rock wool still stands, slightly darkened but structurally intact.
Final Takeaway: Choosing insulation isn't just about thermal performance - it's literally about your life. If you live in a multi-storey block above 3 floors or a public building, rock wool (A1) is the only logical choice. If you use polystyrene (unavoidable in many low-rise homes), make sure there's always a thick render and fire barrier.
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