Some time around 2003, our house received a cleanup and utilities upgrade. Part of that was a layer of blown-in cellulose insulation in the attic and upstairs cathedral ceilings. Unfortunately, the roof was replaced soon after, and a bunch of nails and roofing debris wound up mixed into it, but I collected some of the cleaner material and re-used it by dumping it down the first floor wall cavities from above. I'm going to go buy a few more bales of the stuff and dump it down all the cavities, before installing some 2 x 4 fireblocks at the top of the first floor level. Between the fire-retardant insulation and the fireblock, that should be a pretty good upgrade in terms of fire safety for that part of the house.
But I'm installing foil-backed foam in the ceiling, and I'm sure untreated urethane foam is extremely dangerous and flammable. And what about the urethane spray foam I use to seal it into the framing? I'm using paper-faced fiberglass batts in the exterior walls. I wonder how those all compare?
There are several videos out there, mainly looking at blown-in cellulose vs. fiberglass. One shows an ASTM test comparison, and the cellulose really seems to actually catch fire and much of the surface chars. That seems to be a big contrast to other videos, like the one where a guy melts a penny in his hand with a propane torch, supported by a wad of cellulose. I think the difference is the pre-heating done as part of the ASTM test method. Cellulose, like wood, will decompose to carbon (charcoal) when heated to a high enough temperature. Flammable gasses are given off during this process, and I wonder if it's these gasses burning that you see in the test. I imagine that the cellulose just below the surface looks fine. However, the point still remains, that no matter how much borax or other fire retardant chemicals you treat cellulose with, it's still cellulose, and chemically represents a source of fuel.