Is cellulose insulation fire-resistant?

Yes. It's been shown to slow fire spread. Cellulose is treated with borax during manufacture, and Sustainability Victoria's government guide states that the borax fire-retardant treatment ensures that if the material does ignite, the flame will not spread. Borax melts at around 734°C, roughly 200° above the temperature of a typical house fire, so hold a blowtorch to a handful of treated cellulose and it chars and glows where the flame touches, then stops, rather than carrying the flame across the rest. That's a demonstration you can watch, not a marketing line. It also gives off only carbon dioxide and steam as it chars, rather than the gases that fibreglass batt binders produce. I won't print a fire rating without showing you the test certificate, but the borax chemistry and the government's own wording are why I trust it in a roof. Including my own.

It's the obvious worry, and it's a fair one. Cellulose is recycled paper, and paper burns. The difference is the treatment. The CSIRO describes cellulose fibre as pulverised paper combined with fire-retardant chemicals, and the chemical we use is borax. Borate-treated cellulose has been shown to slow fire spread: Sustainability Victoria's Energy Smart Housing Manual says the borax treatment ensures that if the material does ignite, the flame won't spread. Untreated newspaper in a pile is a fire risk. Borate-treated cellulose, pumped into your ceiling, behaves completely differently: under a blowtorch it chars and glows where the flame touches it, then stops, rather than carrying the flame across the rest of the blanket. The fire test for building materials in Australia is AS 1530.1, which measures ignitability, flame spread, heat and smoke, and borate-treated cellulose is made to be tested against it.

Fibreglass is glass, and glass melts. In a house fire the batts soften and melt away, leaving your roof timbers exposed where you most needed the cover, and the binders that hold the batts together give off gases as they burn. So the cheapest insulation also disappears at the worst possible moment. Cellulose does the opposite: it chars and holds rather than melting, and gives off only carbon dioxide and steam. I'm not telling you cellulose makes a house fireproof; nothing in your ceiling does that. What I'm saying is that when you compare the two materials in the same heat, borate-treated cellulose has been shown to slow fire spread, it stays put and resists carrying a flame, while a batt melts out of the way. That's one more reason it's the only product I'd use in my own home.

There's a well-known demonstration people Google called "the big burn", but be clear that it was a United States test from 1978, not an Australian fire rating, so I'll always present it as a demonstration rather than a certified result. In it, three mock structures were lit, and the write-up reports the fibreglass-insulated building's ceiling collapsed about 21 minutes in, while the cellulose-insulated structure's ceiling held for over an hour and the walls were still standing at the three-hour mark. It lines up with what I see in my own blowtorch test and with what the Australian government sources say about borax stopping flame spread. For the actual Australian numbers, the AS 1530.1 ignitability and flame-spread figures, ask me for the manufacturer's test certificate and I'll show you the real results rather than quote you a rating off the top of my head.