Last night probably everyone were shocked by the terrible fire in the Notre Dame church in Paris, France, which is a World cultural heritage and one of the most loved tourist attraction in Paris.
Hopefully, many of the treasures were saved, and the economic support of the rebuilding has already been impressing, but it will take decades to repair the church. Hopefully with less flammable materials.
The cause of the fire is not yet known but it is likely, that the ongoing repair of the church roof, shown by the bulky scaffold, has played a role.
When I saw the early pictures of the fire, I noticed the yellowbrown smoke evolving, typically for a fire involving polyurethane (PUR) insulation materials. It is not always recognised, in practice, that it is a solid fuel burning very fast.
It is also clear from the picture, that something very flammable is burning inside the rafters in the roof room, and burns before the rafter is in flames, and when the most flammable materials are outburned, then the fire is less violent.
When PUR is used in building insulation, it has to be incapsulated in not-fireable materials, such as concrete, or flame/fire retardants have to be added.
However, flame retardants can only prevent weak ignitors, such as cigarets and may be candle lights to initiate a fire in the materials. The tools workmen use for renovations generate more powerful ignition, e.g. welding flames and angle grinders sparks.
Use of PUR in building insulation is very popular, because it is relatively cheap, easy to work with and efficient heat insulating.
It is very sad, that the non-flammable, but more expensive, insulating materials: glass wool and stone wool were not used instead. Glass wool is in fact a famous French product.
It has been mentioned, that the roof plates consist of metallic lead, which melts already at 327 oC. Probably, the fire also had been less violent with use of copper plates in stead, which melts at 1085 oC.
Finally, it has to be considered, that the smoke from a PUR-fire contains very toxic cyanides; risky especially to the firemen!.