Polyurethane spray foam is pretty damn fiery in most conditions, even if the right number of carcinogenic flame retardants can limit open flame spread on a surface. It's also got the most toxic, thick sort of smoke of the available options, other than the polyiso in Grenfall.
There are two inherently fire-safe, inexpensive insulation options, borated cellulose and mineral wool. You can even use them as fire-stops. Neither are good to use in an application where they receive rain.
Polystyrene and fiberglass get runner up status because they melt & retreat from a fire at sub-ignition temperature, which doesn't spread the flame but does open up the wall.
Fire departments generally love foam because while it does burn it fills entire spaces and prevents fire spread and limits exhaust of combustion byproducts (which limits oxygen intake on the other end). What would have been total losses turn into minor repair and smoke damage rehab. I'm still not a fan though.
> What would have been total losses turn into minor repair and smoke damage rehab.
In my (admittedly minimal) experience as a volunteer firefighter, “minor repair and smoke damage rehab” isn’t a thing once a fire is progresses handheld fire extinguishers.
Water causes a massive amount of damage, and you often need to rip apart walls and floors - with water sometimes joining the party - in order to ensure the fire is fully extinguished.
There are two inherently fire-safe, inexpensive insulation options, borated cellulose and mineral wool. You can even use them as fire-stops. Neither are good to use in an application where they receive rain.
Polystyrene and fiberglass get runner up status because they melt & retreat from a fire at sub-ignition temperature, which doesn't spread the flame but does open up the wall.