Is spray foam toxic, or does it off-gas?

The real, documented concern is during spraying and curing, not for years afterwards. Spray polyurethane foam is made by reacting isocyanates on site, and SafeWork NSW lists isocyanates as a known cause of occupational asthma, with sensitised workers reacting at even very low or brief exposure. That's why installers wear full respiratory protection and occupants are typically asked to leave the home while it sprays and cures, and why cheap or badly-mixed foam can release fumes for days while it sets. The fair caveat, so this stays honest: once it has fully cured and been correctly installed, the foam is largely inert. So it's not a product I'd call dangerous to live with afterwards, it's the install-and-cure window where the care is needed. Cellulose carries none of that chemistry: it's recycled paper and borax, inert from day one, though like any loose-fill we wear dust masks while we pump it.

Because of the chemistry while it's going on, not because of what's left afterwards. Spray foam is mixed and reacted right there in your roof from two liquid components, one of them an isocyanate, and during that reaction and cure it releases fumes and fine airborne particles. SafeWork NSW lists isocyanates as a known cause of occupational asthma (sensitised workers can react at even very low exposure), so the installers spray in full respiratory protection, and the standard advice is for occupants, and pets, to clear out of the home until it has cured and the air has cleared, often a day or more. That's a sensible precaution, and any reputable foam installer will tell you the same. It's worth understanding before you book, because it's not how the other insulations work: with pump-in cellulose there's no chemical reaction in your roof and no need to move out. We wear dust masks for the loose fibre, you stay put.

On the honest evidence, no. Properly cured, correctly-installed foam is largely inert, and that's the caveat I'll always give so I'm being fair to the product. The off-gassing question is really an install-and-cure-phase question: the isocyanate reaction and any VOCs happen as it's sprayed and sets, which is why occupants vacate during that window. The exceptions you read about, a smell that lingers for weeks, or ongoing complaints, generally trace back to foam that was mixed wrong, sprayed too thick in one pass, or applied in the wrong conditions, so it never cured properly. That's a workmanship risk, not a feature of the material. I don't sell foam, so I've no reason to talk it up or down: the straight answer is that the cure phase is where the care matters, and a job done right shouldn't leave you with an ongoing fume problem.

No. There's no chemical reaction when we pump cellulose. It's recycled paper fibre treated with borax, a natural mineral salt, and it's inert from the moment it goes in. There's nothing to cure, no isocyanates, no two-part mix reacting in your roof, so there's no off-gassing window and no reason for you to leave the house. The one honest thing to flag is dust: like any loose-fill, cellulose can be dusty going in, so our installers wear dust masks while they pump it, which the CSIRO notes is the sensible handling precaution for loose-fill. But that's airborne paper dust during the install, not a chemical fume, and it settles. That difference, a recycled plant fibre you live with from day one versus a chemical reaction you move out for, is one of the reasons I'd put cellulose in my own home before foam.