Controlling Cladding Fires With External Sprinklers
There have been several disastrous tower fires that were propagated upward by flammable sidings. Of course, all fire propagates upward, since hot air rises. The dangerous siding is composed of aluminum sides with flammable polyethylene filling. The purpose of the sidings is sometimes looks and also for insulating the warmth of buildings to save energy. Since aluminum is a metal, it is highly conductive, and not insulating. That is why the plastic insulator is included in the middle. There was also a fire resistant core cladding, but this was not chosen.
Time for my usual disclaimers: I am not an engineer or combustion expert, nor am I familiar with the cladding in question more than I have read in the articles on the tower fires.
I had first thought of cheaply covering the surface with a fire resistant material. However, aluminum originally coats itself with aluminum oxide, which is fire resistant. Aluminum does not melt until 1221 degrees F. This leads to the expectation that if you could cool the surface with evaporating water, it would be the first barrier, and when the exposed polyethylene catches fire, the water could cover it and keep the flames away.
Polyethylene actually melts at 248-356 degrees F or 120-180 degrees C for high density, and at 221-239 degrees F, or 105-115 degrees C at low density. It didn’t seem apparent that the panels were sealed. Thus the polyethylene would first melt out of the protecting aluminum and then catch fire.
In the Grenfell Tower fire, there was flammable Celotex insulation in the building, and this was not supposed to be paired with a flammable cladding.
While installing a ceiling water system in retrofitting a building can be expensive, proportional to the floor areas, installing an external perimeter system is proportional to the exterior length or circumference of the building, which is only proportional to the width of a building.
The external system should be repeated every few number of floors to limit the number of floors affected. Clearly this should be tested with full sized cladding on skeleton buildings, or buildings about to be demolished. Different models of distributing the water, degree of misting and intensity of flow can be tested. A smart watering system could choose how much of the circumference has to be covered in a particular fire, and then provide more concentrated delivery and flow.
While actual buildings have to be modified to make sure that the external fire cannot come into the building, interior water systems can be installed on the same floors as the external system, to make sure that the fire is totally blocked at those floors.
While replacing flammable siding is a start on a fully preventative system, the separation of the inside of the building from the outside is still required to prevent the fire from growing upward and across the building.
The safeguarding of older cladding building is not proceeding rapidly enough. The external watering system can be a rapid temporary step before the building can be fully safeguarded.
