From reaction-to-fire class A1 to fire resistance EI 120

On New Year’s Day 2026, in a bar in Crans-Montana, Switzerland, sparks from fireworks set off in bottles of champagne struck the ceiling of an underground room. According to the findings of the investigation to date, the fire found fuel in the soundproofing lining of the low ceiling and, in a matter of moments, the entire room was ablaze. Forty people lost their lives and 116 were injured, most of them seriously.

The second issue of Building Envelope Notes is devoted to a question that concerns not decoration but life: when a fire breaks out in an enclosed assembly space, what aspects of the building’s structure itself determine whether people will be able to escape in time?

A recurring theme

The tragedy in Crans-Montana was not unprecedented; it was, unfortunately, all too familiar. The same scenario has been recorded time and time again over decades, with almost identical characteristics: a source of ignition (often fireworks), flammable foam cladding on walls and ceilings, low ceiling height, overcrowding and blocked escape routes.

The most thoroughly studied example is the fire at The Station nightclub (Rhode Island, USA, 2003), which was thoroughly investigated by the US National Institute of Standards and Technology (NIST). Fireworks ignited polyurethane foam that had been installed as sound insulation around the stage. The smoke reached the exits in just over a minute; flashover occurred in about one and a half minutes. One hundred people died, most not from the flames, but because they were unable to escape in conditions that had already become untenable. In NIST’s laboratory tests, the non-fire-rated foam ignited in 3 to 14 seconds; a sample containing a flame retardant, under the same conditions, did not ignite at all.

The conclusion reached by fire engineers is consistent: in these spaces, it is not the fire itself that kills you first, but the speed at which it spreads and the smoke. Both are determined by the materials used on the walls and ceiling.

Active and passive fire protection, and why both are necessary

The Building Fire Safety Regulations distinguish between two categories. Active fire protection comprises the systems that detect, signal and combat fire: fire detection, fire alarms, sprinkler systems, smoke control. Passive fire protection is incorporated into the structure itself: the fire resistance of structural elements, the fire behaviour of surfaces, fire compartments, and fire-protected escape routes.

The lesson to be learnt from these fires is right here. Active systems depend on operating at the right moment, and are often missing, out of service or simply too slow to react. When the building’s own cladding is combustible, no sprinkler system can keep up with a fire that reaches flashover within one and a half minutes. Passive fire protection is the only line of defence that does not need to be ‘activated’: either it is built into the walls and ceiling, or it is not there at all.

Fire behaviour: the material’s classification determines the first few minutes

Fire behaviour describes the extent to which a material contributes to a reaction to fire. The European standard EN 13501-1 classifies construction products into classes from A1 to F: A1 and A2 are practically non-combustible (minerals, glass, stone wool, cementitious products), whilst materials from B to F are combustible, with an increasing contribution to the fire.

The point that is often overlooked is that the letter is not the only factor. From class A2 and below, it is accompanied by two indices which, in a gathering space, carry as much weight as the letter itself:

• The ‘s’ index (smoke): s1 = minimal, s2 = moderate, s3 = intense. This is crucial, because in most deaths occurring in enclosed spaces, the cause is toxic smoke and gases (carbon monoxide, hydrogen cyanide from foams), not burns. A material may be difficult to ignite yet still produce lethal smoke.
• The d-index (flaming droplets): d0 = none, d1 = limited, d2 = free fall of flaming material, which may cause burns and spread fire.

That is why the essential specification for ceilings and cladding in high-risk areas is not simply ‘non-combustible’, but a full class designation such as A1 or A2-s1,d0: non-combustible, with minimal smoke emission, and no flaming droplets. Class A1 is not accompanied by the s and d indices, as the material is considered to make no contribution whatsoever to the fire. Flammable polyurethane foam is the antithesis of this requirement.

Fire resistance: the time you have to evacuate

Whilst fire behaviour relates to the surface, fire resistance (EN 13501-2) relates to the ability of a structural element to withstand fire for a specified period of time. It is expressed by three criteria: R (load-bearing capacity), E (integrity, i.e. preventing the passage of flames and hot gases) and I (thermal insulation capacity, i.e. resistance to heat transfer). This gives rise to the familiar ratings, e.g. EI 60, EI 90, EI 120.

The figure is in minutes, measured in a laboratory under the standardised fire curve (ISO 834). A real fire may be more intense, so the rating should be regarded as a minimum guarantee, not as a stopwatch. The principle, however, remains the same: an EI 90 escape wall keeps fire and smoke on the other side for much longer than it takes for a space to be evacuated and for the fire service to arrive. In assembly spaces, where the speed of evacuation is paramount, this time is literally the difference.

The Greek regulatory framework: Presidential Decree 41/2018

In Greece, the Building Fire Safety Regulations (Presidential Decree 41/2018) are in force, which came into force in August 2018, replacing the previous regulation of 1988 and bringing it into line with European standards. Public assembly spaces are defined as those where at least 50 people gather for social, cultural, recreational and related activities.

The regulation is based precisely on the following pair of standards: building products are classified according to their reaction to fire in accordance with EN 13501-1, whilst structural elements (load-bearing and non-load-bearing) are classified according to their fire resistance in accordance with EN 13501-2. It regulates interior finishes, escape routes, fire compartments and fire resistance ratings, and requires a combination of active and passive measures alongside the appropriate choice of materials. The exact classes and ratings required depend on the category of use, the theoretical occupancy and the geometry of the space, and must always be verified in the relevant specific section of the regulations for that particular use.

From design to construction

The application of all these principles to actual dry construction projects involves five levels:

• High-risk ceilings and cladding: Class A1 non-combustible boards, which add no thermal load whatsoever. Knauf Fireboard is one such A1 solution in our range, featuring glass fibre mesh instead of paper on its surfaces. There are other non-combustible A1 materials, e.g. calcium silicate boards; what matters is the certified class, not the brand name.
• Non-combustible acoustic ceilings: the need for sound absorption was the reason for the use of flammable foam in both fires. The same requirement is met by perforated plasterboard and mineral-based acoustic panels, which provide high sound absorption without becoming a fire hazard overhead. See the solution: CLEANEO® AKUSTIK perforated plasterboard.
• Walls, partition walls and escape routes: high fire-resistance plasterboard systems (e.g. Knauf Diamant, Knauf Ultra Board), which achieve, depending on the tested configuration, EI ratings of 60, 90 or 120. Please note: fire resistance is a property of the system, not of the individual board. Only the specific, certified configuration (board + stud frame + insulation + fixing method) bears the rating; which is why the classification report is what counts, not a material’s product leaflet. You will find an example of a cross-section in the solution ‘Sound insulation of partition walls using dry construction’.
• Wall core: stone wool, which is naturally non-combustible, with a melting point above 1000°C. It acts as a thermal barrier within the frame, preventing the transfer of heat to the adjacent space; air within an uninsulated cavity has the opposite effect.
• Joints and penetrations: a fire-rated structure is only as strong as its weakest link. If slab joints and pipe and cable penetrations are not sealed with certified fire-rated materials, fire and smoke will find a way through. Sealing is not a minor detail; it is a prerequisite for the system’s overall rating to be valid.

The conclusion

Choosing cheap, flammable materials for ‘decoration’ or ‘acoustics’ is not a cost-saving measure; it is an additional fire and thermal load hanging over people’s heads. On the contrary, a properly designed passive fire protection system, featuring non-combustible surfaces and certified fire-rated systems, provides both the aesthetic and acoustic qualities required by the space, without turning it into a fuel source.

Active fire protection provides the warning; passive fire protection provides the time. Both are needed, but only the latter is built to ensure it won’t let you down.

If you are designing or constructing such a space, please send me the project details and we will get back to you with recommended certified systems and the corresponding fire resistance ratings.

Dimitris Argyriou, ARGYNET

Sources & bibliography

• NIST NCSTAR 2, Report of the Technical Investigation of The Station Nightclub Fire, National Institute of Standards and Technology (USA), 2005.
• EN 13501-1: Classification of construction products with regard to their reaction to fire (Euroclasses A1–F, s and d indices).
• EN 13501-2: Classification of structural elements in terms of fire resistance (R, E, I criteria).
• ISO 834-1: Fire resistance tests for building elements; standardised temperature–time curve.
• Presidential Decree 41/2018 ‘Regulations on Fire Protection of Buildings’ (Government Gazette 80/A/7 May 2018).
• Regulation (EU) No 305/2011 (CPR) and Delegated Regulation (EU) 2016/364 on the classification of reaction to fire.
• Valais Cantonal Police, announcements and press conferences regarding the fire at Le Constellation, Crans-Montana (1–5 January 2026).
• Associated Press, «Fire at a bar in the Swiss Alps left people dead, injured», 1 January 2026.